THX for the answer.
Actually I finally found out that on my PC an old local file version was stored, which the app automatically tried to open at startup (unfortunately there does not seem to be an option to choose a certain file at startup?). Luckily I remembered the master password for this one and could open it. After resetting the PC data then in the menu I could import a backup, which I could create with my working iPad version and afterwards activated the sync with google drive again.
So finally it seems that it is working :-)