I have checked that before and did doveconf -n -c... at first but the result was same. After that I have removed Postfix, Dovecot, saslauthd and all configurations. I installed everything again (by following documentations for Dovecot and Postfix) and the problem is still here.
To me it looks like that there is a problem when dovecot tries to connect to PAM but I'm not sure...
1.x to 2.x is a *major upgrade* so look in your syslog (not the maillog) and the next time before oyu do a migration i suggest https://www.google.at/search?q=upgrade+dovecot+1.x+to+2.x which will result at the following instructions *before* start the upgrade http://wiki2.dovecot.org/Upgrading/2.0
