[Dovecot] authn_name and authz_name differ: not supported

Hello,

I'm trying to authenticate using GSSAPI, but getting this in dovecot.log "authn_name and authz_name differ: not supported". What is actually trying to say me? I've remeber once encounter this problem but it get away silently. I'm using Mozilla Thunderbird 3 beta 3 and Dovecot 1.0.15