Variable expansion in passwords

Hello list

A user reports failure to log in after password change. The client side error message indicates a lost or dropped connection by the server.

A peek into the server logs reveales this: dovecot: imap(user@example.com)<53267><el0HoZGqHIdb0K2G>: Error: Failed to expand plugin setting plain_pass = 'asdf%?asdf': Unknown variable '%?'

This looks a lot like a %-variable in the user provided password is tried to be expanded... Is this a bug or did I misconfigure something?

output of dovecot -n is attached.

regards lukn