I'm have the same problem with Centos 5.6 Plesk 10.3.0.
In maillog i see:
Jan 24 14:00:26 host2 imapd: IMAP connect from @ [::ffff:127.0.0.1]INFO: LOGIN, user=example@domain.com, ip=[::ffff:127.0.0.1], protocol=IMAP
Jan 24 14:00:26 host2 imapd: 1327406426.717296 DISCONNECTED, user=exampe@domain.com, ip=[::ffff:127.0.0.1], headers=0, body=0, rcvd=45, sent=418, maildir=/var/qmail/mailnames/domain.com/example/Maildir
At atmail access.log I see:
[24/Jan/2012:14:00:26 +0200] "GET /parse.php?file=html/login-light.html HTTP/1.1" 200 4193 "http://webmail.domain.com/index.php?error=auth" "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.7 (KHTML, like Gecko) Chrome/16.0.912.75 Safari/535.7"
Looks that error with authentification - "http://webmail.domain.com/index.php?error=auth"
But in web i didn't saw any error about wrong username or something else, just page reloads with login page. If i enter wrong pass, then i get error.