This hits a ASUS dongle with Bus 001 Device 004: ID 0b05:184c ASUSTek Computer, Inc. 802.11ac NIC
which is also WLAN (WLAN/BT).

The device can see the other but cannot pair. Neither is a window opened when the PC tries to connect to the device (pairing aborts prematurely), nor is it opened when a device tries to connect to the PC (no device found) although the BT device is shown with name. 

On Leap 15.4 the very same device pairs flawlessly. So the issue is restricted as it appears to TW. 

in journalctl I find the protocol of "timeout":
 16:43:19 localhost bluedevil-wizard[19224]: kf.bluezqt: PendingCall Error: "Authentication Timeout"
set 30 16:43:03 localhost systemd[6683]: Started Aggiungi dispositivo Bluetooth - Aggiungi dispositivo Bluetooth.
set 30 16:42:22 localhost akonadi_pop3_resource[7978]: DIGEST-MD5 common mech free
set 30 16:42:20 localhost akonadi_pop3_resource[7976]: DIGEST-MD5 common mech free
set 30 16:41:08 localhost systemsettings[19149]: Qt Quick Layouts: Detected recursive rearrange. Aborting after two iterations.
set 30 16:41:08 localhost systemsettings[19149]: Qt Quick Layouts: Detected recursive rearrange. Aborting after two iterations.
set 30 16:41:08 localhost systemsettings[19149]: Qt Quick Layouts: Detected recursive rearrange. Aborting after two iterations.
set 30 16:41:08 localhost systemsettings[19149]: Qt Quick Layouts: Detected recursive rearrange. Aborting after two iterations.
set 30 16:41:08 localhost systemsettings[19149]: Qt Quick Layouts: Detected recursive rearrange. Aborting after two iterations.
set 30 16:41:08 localhost systemsettings[19149]: Qt Quick Layouts: Detected recursive rearrange. Aborting after two iterations.
set 30 16:41:08 localhost systemsettings[19149]: QQmlEngine::setContextForObject(): Object already has a QQmlContext
set 30 16:41:08 localhost systemsettings[19149]: file:///usr/lib64/qt5/qml/org/kde/kirigami.2/ScrollablePage.qml:200:9: QML MouseArea: Binding loop detected for property "width"
set 30 16:41:08 localhost systemsettings[19149]: file:///usr/lib64/qt5/qml/org/kde/kirigami.2/ScrollablePage.qml:200:9: QML MouseArea: Binding loop detected for property "width"
set 30 16:41:08 localhost systemd[6683]: Started Impostazioni di sistema - Impostazioni di sistema.
set 30 16:40:35 localhost plasmashell[6920]: file:///usr/share/plasma/plasmoids/org.papylhomme.diskmonitor/contents/ui/PopupDialog.qml:49: TypeError: Cannot read property 'right' of null
set 30 16:40:35 localhost plasmashell[6920]: file:///usr/share/plasma/plasmoids/org.papylhomme.diskmonitor/contents/ui/PopupDialog.qml:48: TypeError: Cannot read property 'left' of null
set 30 16:40:35 localhost plasmashell[6920]: file:///usr/share/plasma/plasmoids/org.papylhomme.diskmonitor/contents/ui/PopupDialog.qml:49: TypeError: Cannot read property 'right' of null
set 30 16:40:35 localhost plasmashell[6920]: file:///usr/share/plasma/plasmoids/org.papylhomme.diskmonitor/contents/ui/PopupDialog.qml:48: TypeError: Cannot read property 'left' of null
set 30 16:40:35 localhost plasmashell[6920]: file:///usr/share/plasma/plasmoids/org.papylhomme.diskmonitor/contents/ui/PopupDialog.qml:49: TypeError: Cannot read property 'right' of null
set 30 16:40:35 localhost plasmashell[6920]: file:///usr/share/plasma/plasmoids/org.papylhomme.diskmonitor/contents/ui/PopupDialog.qml:48: TypeError: Cannot read property 'left' of null
set 30 16:40:35 localhost plasmashell[6920]: file:///usr/share/plasma/plasmoids/org.papylhomme.diskmonitor/contents/ui/PopupDialog.qml:49: TypeError: Cannot read property 'right' of null
set 30 16:40:35 localhost plasmashell[6920]: file:///usr/share/plasma/plasmoids/org.papylhomme.diskmonitor/contents/ui/PopupDialog.qml:48: TypeError: Cannot read property 'left' of null
set 30 16:40:35 localhost plasmashell[6920]: file:///usr/share/plasma/plasmoids/org.papylhomme.diskmonitor/contents/ui/PopupDialog.qml:49: TypeError: Cannot read property 'right' of null
set 30 16:40:35 localhost plasmashell[6920]: file:///usr/share/plasma/plasmoids/org.papylhomme.diskmonitor/contents/ui/PopupDialog.qml:48: TypeError: Cannot read property 'left' of null
set 30 16:43:28 localhost dbus-daemon[1608]: [system] Rejected send message, 0 matched rules; type="error", sender=":1.277" (uid=1000 pid=7050 comm="/usr/bin/wireplumber") interface="(unset)" member="(unset)" error name="org.bluez.MediaEndpoint1.Error.NotImplemented" requested_reply="0" destination=":1.2" (uid=0 pid=1607 c>
set 30 16:43:28 localhost dbus-daemon[1608]: [system] Rejected send message, 0 matched rules; type="error", sender=":1.277" (uid=1000 pid=7050 comm="/usr/bin/wireplumber") interface="(unset)" member="(unset)" error name="org.bluez.MediaEndpoint1.Error.NotImplemented" requested_reply="0" destination=":1.2" (uid=0 pid=1607 c>
set 30 16:43:28 localhost dbus-daemon[1608]: [system] Rejected send message, 0 matched rules; type="error", sender=":1.277" (uid=1000 pid=7050 comm="/usr/bin/wireplumber") interface="(unset)" member="(unset)" error name="org.bluez.MediaEndpoint1.Error.NotImplemented" requested_reply="0" destination=":1.2" (uid=0 pid=1607 c>
set 30 16:43:28 localhost dbus-daemon[1608]: [system] Rejected send message, 0 matched rules; type="error", sender=":1.277" (uid=1000 pid=7050 comm="/usr/bin/wireplumber") interface="(unset)" member="(unset)" error name="org.bluez.MediaEndpoint1.Error.NotImplemented" requested_reply="0" destination=":1.2" (uid=0 pid=1607 c>
set 30 16:43:28 localhost dbus-daemon[1608]: [system] Rejected send message, 0 matched rules; type="error", sender=":1.277" (uid=1000 pid=7050 comm="/usr/bin/wireplumber") interface="(unset)" member="(unset)" error name="org.bluez.MediaEndpoint1.Error.NotImplemented" requested_reply="0" destination=":1.2" (uid=0 pid=1607 c>
set 30 16:43:28 localhost dbus-daemon[1608]: [system] Rejected send message, 0 matched rules; type="error", sender=":1.277" (uid=1000 pid=7050 comm="/usr/bin/wireplumber") interface="(unset)" member="(unset)" error name="org.bluez.MediaEndpoint1.Error.NotImplemented" requested_reply="0" destination=":1.2" (uid=0 pid=1607 c>
set 30 16:43:28 localhost dbus-daemon[1608]: [system] Rejected send message, 0 matched rules; type="error", sender=":1.277" (uid=1000 pid=7050 comm="/usr/bin/wireplumber") interface="(unset)" member="(unset)" error name="org.bluez.MediaEndpoint1.Error.NotImplemented" requested_reply="0" destination=":1.2" (uid=0 pid=1607 c>
set 30 16:43:28 localhost dbus-daemon[1608]: [system] Rejected send message, 0 matched rules; type="error", sender=":1.277" (uid=1000 pid=7050 comm="/usr/bin/wireplumber") interface="(unset)" member="(unset)" error name="org.bluez.MediaEndpoint1.Error.NotImplemented" requested_reply="0" destination=":1.2" (uid=0 pid=1607 c>
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSource/opus_05_duplex
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSink/opus_05_duplex
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSource/opus_05
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSink/opus_05
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSource/faststream_duplex
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSource/faststream
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSource/aptx_ll_duplex_0
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSource/aptx_ll_duplex_1
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSource/aptx_ll_0
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSource/aptx_ll_1
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSource/sbc_xq
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSink/sbc_xq
set 30 16:43:28 localhost systemd[1]: Starting Load/Save RF Kill Switch Status...
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSource/sbc
set 30 16:43:28 localhost systemsettings[19149]: file:///usr/share/kpackage/kcms/kcm_bluetooth/contents/ui/General.qml:35: TypeError: Cannot read property 'name' of null
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSink/sbc
set 30 16:43:28 localhost systemsettings[19149]: file:///usr/share/kpackage/kcms/kcm_bluetooth/contents/ui/General.qml:41: TypeError: Cannot read property 'address' of null
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSource/aac
set 30 16:43:28 localhost systemsettings[19149]: file:///usr/share/kpackage/kcms/kcm_bluetooth/contents/ui/General.qml:47: TypeError: Cannot read property 'powered' of null
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSink/aac
set 30 16:43:28 localhost systemsettings[19149]: file:///usr/share/kpackage/kcms/kcm_bluetooth/contents/ui/General.qml:53: TypeError: Cannot read property 'discoverable' of null
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSource/aptx
set 30 16:43:28 localhost systemsettings[19149]: file:///usr/share/kpackage/kcms/kcm_bluetooth/contents/ui/General.qml:24:9: Unable to assign [undefined] to QObject*
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSink/aptx
set 30 16:43:28 localhost systemsettings[19149]: file:///usr/share/kpackage/kcms/kcm_bluetooth/contents/ui/General.qml:24:9: Unable to assign [undefined] to QObject*
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSource/aptx_hd
set 30 16:43:28 localhost systemd-udevd[1083]: /etc/udev/rules.d/55-brother-libsane-type1-inst.rules:14 Invalid key 'SYSFS'.
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSink/aptx_hd
set 30 16:43:28 localhost systemd-udevd[1083]: /etc/udev/rules.d/55-brother-brscan4-libsane-type1.rules:9 Invalid key 'SYSFS'.
set 30 16:43:28 localhost bluetoothd[1607]: Endpoint unregistered: sender=:1.277 path=/MediaEndpoint/A2DPSource/ldac
set 30 16:43:28 localhost plasmashell[6920]: file:///usr/lib64/qt5/qml/org/kde/plasma/extras/PlaceholderMessage.qml:238:5: QML Heading: Binding loop detected for property "verticalAlignment"
set 30 16:43:28 localhost plasmashell[6920]: file:///usr/lib64/qt5/qml/org/kde/plasma/extras/PlaceholderMessage.qml:238:5: QML Heading: Binding loop detected for property "verticalAlignment"
set 30 16:43:28 localhost kernel: usb 1-1.7: USB disconnect, device number 6
set 30 16:43:19 localhost bluedevil-wizard[19224]: kf.bluezqt: PendingCall Error: "Authentication Timeout"
set 30 16:43:03 localhost systemd[6683]: Started Aggiungi dispositivo Bluetooth - Aggiungi dispositivo Bluetooth.


So it is currently not possible to pair certain devices (in particular ones following older BT protocols (here a Samsung GPS/UMTS phone). 

I doubt the problem is only given by the dongle, it appears it also hits other hardware BT like Cambridge Silicon Radio BT dongle. So changing dongle does not change the issue in TW.