Tried it again using CLI, this time successful. Below is info of what I did, maybe it can help somebody.
It seems there was some rudiment from my gui tries which prevented the CLI attempt to work on my first try.
On Libreelec over ssh:
Input:
bluetoothctl
scan on
trust XX:XX:XX:XX:XX:XX
pair XX:XX:XX:XX:XX:XX
Output:
...
[agent] PIN code: XXXXXX
...
On Keyboard:
XXXXXX Enter
Output:
...
Pairing successful
...
Problem: Adding a bluetooth keyboard works until the code to enter in the keyboard for pairing is shown. The numbers are invisible in the GUI. Additionally I was not able to enter any numbers and press return, the GUI is locked in that state. I tried it on the CLI, which allowed me to see the pairing code numbers, but it's not possible to enter them on the keyboard. Even when using the CLI the GUI tries to show the code field with the invisible numbers.
Internal bluetooth is disable via dtoverlay=disable-bt in config txt. Using CSR8510 external BT dongle instead because of metal case.
LibreELEC:~ # cat /sys/firmware/devicetree/base/model
Raspberry Pi 4 Model B Rev 1.4L
LibreELEC:~ # dmesg | grep CSR
[ 2.317830] usb 1-1.3: Product: CSR8510 A10
[ 267.698120] usb 1-1.3: Product: CSR8510 A10
LibreELEC:~ # lsusb
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 001 Device 004: ID 046d:c52b Logitech, Inc. Unifying Receiver
Bus 001 Device 005: ID 0a12:0001 Cambridge Silicon Radio, Ltd Bluetooth Dongle (HCI mode)
Bus 001 Device 002: ID 2109:3431 VIA Labs, Inc. Hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
