Just to confirm, did you try Tools>Check Database? If it didn’t help, try the following:
- Find a card triggering the error.
- Open the card in the browser and find its note type.
- Search for all cards using that note type (you can do that from the sidebar) and use Notes>Change Notetype to switch them to a different note type.
- Delete the problematic notetype from your collection from Tools>Manage Notetypes.