Got this issue fixed by disabling SSL. Now I'm experiencing error "failure to connect to imap server" when trying to log into roundcube.
Edit: Errors in the dovecot.log:
Jun 14 09:28:15 imap(postmaster@matterhost.com): Error: Invalid user settings. Refer to server log for more information.
Jun 14 09:31:13 imap-login: Info: Login: user=<postmaster@matterhost.com>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=3466, secured, session=<FHRt6BnfiAB/AAAB>
Jun 14 09:31:13 imap(postmaster@matterhost.com): Error: chdir(/var/vmail/vmail1/matterhost.com/p/o/s/postmaster-2013.06.14.08.47.22//) failed: Permission denied (euid=2000(<unknown>) egid=2000(<unknown>) missing +x perm: /var/vmail, dir owned by 5000:5000 mode=0700)
Jun 14 09:31:13 imap(postmaster@matterhost.com): Error: chdir(/var/vmail/vmail1/matterhost.com/p/o/s/postmaster-2013.06.14.08.47.22/) failed: Permission denied
Jun 14 09:31:13 imap(postmaster@matterhost.com): Error: user postmaster@matterhost.com: Initialization failed: Namespace '': stat(/var/vmail/vmail1/matterhost.com/p/o/s/postmaster-2013.06.14.08.47.22//Maildir) failed: Permission denied (euid=2000(<unknown>) egid=2000(<unknown>) missing +x perm: /var/vmail, dir owned by 5000:5000 mode=0700)
Jun 14 09:31:13 imap(postmaster@matterhost.com): Error: Invalid user settings. Refer to server log for more information.