Maybe a bug in Mailcow then? Might be worth the time to submit an issue in GitHub. Logs from the containers might also give some clues as to what is wrong (and will likely be needed as part of logging the issue.)
To rule out an obscure OS / end-user-device issue with Mailcow (especially since you mentioned an M1 Mac which has its own fingerprint support), you might want to try again from a different machine (e.g. Windows or an Intel Mac) and see if it works?