Cannot send any emails / answers

Hi, I've just updated to Thunderbird 91 (x64, Windows 10 21H1) and Cardbook 70.6.

Since then, I cannot send or answer any emails. I can type the text, but after clicking "Send", the receiver and topic lines change to grey/inactive. And nothing happens, the mail message window stays alive and does not disappear. But the receiver's mail adress is marked with a point and says "receiver not contained in the adress book". Which might be true for the Thunderbird adress book, but not for cardbook, as the receiver is contained in there.

After deactivating cardbook, sending and answering works fine (despite the fact that all my adressbook data is not available).