Welcome to the forum 
I see that you’ve already posted in a German language topic for the same problem. I’m sure the moderators will find a solution which will allow you to join the conversation on this somewhat specific subject. You can of course obtain translations with a service such as deepl.com .
Yoir first action should be to notify official Fairphone support. https://support.fairphone.com
With Bluetooth devices generally the first things I would do are:
- check whether any software update is available for the device
- delete the pairing both on the phone and on the device
- make sure that the apps are allowed to run in the background
Have you encountered problems with other BT devices?
Downgrading is only possible at the expense of all the data on the phone, comes with risks and is rarely if ever a viable long term solution.
P. S. Could you specify the new OS version currently on your phone. Go to
Settings > About phone > Build number