mutt/tls : prime sent is too short

Hi, I just upgraded my TW notebook to 20210316. Now I cannot log in to my (university) mail server via mutt anymore. I only get gnutls_handshake: The Diffie-Hellman prime sent by the server is not acceptable (not long enough). Looks some default settings have changed? But which - and how do I override it? Thanks for any hints!