[Dovecot] connection dropped by imap

hello,

when i create imap account in mail client (ex: outlook, thunderbird) it's allright. but if i want login with this account to webmail (i have 2 webmail: squirrelmail and roundcube) commonly i get error. squirrelmail say: Connection droped by imap. roundcube say nothing(white screen). But sometimes i can login. and i see one thing... when i restart apache, i can login above 1-2 hours, but later i can't login. i restart apache again and again i can login 1-2 hours.

please help