One more thing, that might (or might not) be important here: after I reverted to 1.7.2-1 now, I left set imap_authenticators="login" in the config. I started seeing the same error with 1.7.2-1 that I saw with the newer versions, but only with the mailserver, that also shows the error with the newer mutt versions. The other one, which works with the newer versions, also works with the set imap_authenticators="login" in the config.
I deleted the entry in the config and all was well again with mutt 1.7.2-1 Cheers Armin
