the error message you're getting is just an elaborate way of saying
"login failed". judging by the mozilla bug, the problem might be that
you're using different capitalization of your login name.
I don't get that from the mozilla bug, they mention capitalization of
mechanisms, but not of logins, see for instance this diff:
https://bugzilla.mozilla.org/attachment.cgi?id=351=diff
where they replace "login" and "plain" by "LOGIN" and "PLAIN".
alternatively, the login *is* wrong, because the password expired or
something like that.
if neither applies, try configuring "AuthMech LOGIN".
AuthMech LOGIN works indeed (so my logins and passwords are correct).
