@mxmehl This happens on https://accounts.bahn.de/auth/realms/db/account/ when changing from a non-single-letter local part address to one that has a one-letter local part. Similarly this happens, when registering a new account on accounts.bahn.de - the form does a live validation of the email and rejects it with a red error message saying it's invalid, then grays out the button to continue.

Thanks for following-up, much appreciated (I understand there's no promises, of course.).