I managed to get the mouse to work. I cannot say exactly what made the difference. Here is what I did:
I plugged in another USB dongle because I knew it worked with manjaro and the mouse. I also rebooted the laptop (Though I’m sure I did this in the meantime). After the reboot I searched for devices using the standard Bluetooth Application in KDE using the dongel as the interface. I was able to find and connect to the mouse. After that I removed the dongel and searched again using the built-in interface. Suddenly the mouse was there. I have no idea what made the difference but I can use it now.