[Dovecot] connection dropped by imap

[Mantas]

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