I have done some additional testing:
I have used a second bluetooth mouse. The same thing happens. Reboot when trying to connect
Also I noticed, that when I wait until all bluetooth devices have been found and I connect to a bluetooth device, the sounds in the menu stop working. So just to clarify this: This is without the reboot (as mentioned in the post above).
And the sound is connected to the TV through HDMI-cable. So it is very weird that the sounds of the menu stop working when connecting a bluetooth device, but the sound of the movie is working without a problem.
Hope this helps.
If you guys need any more information, just let me know.