hey @mXm-Jochen,
Sorry for the trouble you are going through.
Please follow these troubleshooting steps:
I. App downloaded from Mac App store:
Quit Enpass (If running).
Go to User --> Library Folder--> Containers --> in.sinew.Enpass-Desktop --> Data folder--> Library--> walletx.db--> Rename it as oldwalletx.db.
NOTE: Usually Library folder is hidden, so make it visible by pressing and holding the Option key.
II. App downloaded from Enpass website:
Quit Enpass.
Go to User Documents --> Enpass folder--> Rename it as OldEnpass.
Hope this helps!