Vivoactive 3 stops responding after trying to stop the sleep monitor


My Vivoactive 3 is doing nothing now anymore! I connected it to the app, pressed ‘Test Sensor’ and stopped the sleep tracking on my phone. My watch just says ‘Stopping…’ On the first try I could reset the watch pressing the power button 15secs but after the 2nd try the watch doesn’t respond to this too.
Seems something is wrong there

Now I could restart the watch somehow, so I tried it another time.
This time I created an alarm for the next 3 minutes and started sleep monitoring.
After 3 minutes I validated the alarm on my phone but on the watch its still written ‘Tracking’ and I can’t reset the watch again…