Fritz!Fon C4 stays white or flickers

I have here a Fritz!Fon C4 (a softphone) with updated firmware. If I know insert batteries, the screen sometimes turn white or flickers but the phone seems to start and connect but remains unusable.

Any ideas besides removing battery and re-inserting it? I already filled a ticker with #AVM.
#AVM avm bug c4 problem softphone
So far not much progress. Also their suggestions on how to fix it did not work, the display remain ~99.9% white and only one time after a reset, I saw the blue start screen which became mixed up again after some seconds.

So I decided to wrote a complain to them.
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