Timo,
Is the director proxy inactivity timeout configurable? I just recently attempted to switch our production webmail's imapproxy system to use a pool of two directors and ran into problems with it apparently disconnecting them unexpectedly. I don't believe that it IDLEs, I think it just UNSELECTs to reset the state and then keeps the connection open until it's used again, or until a configurable delay has elapsed, after which it logs out on its own.
I've got the imapproxy inactivity delay set to 300 seconds, and it seems like the director is disconnecting them before that time is up. I haven't had the same problem when imapproxy connects directly to the backend servers, which are running 1.2.12.
On the webmail system, imapproxy is logging a LOT of: Jul 11 21:10:21 cc-mailapps1 in.imapproxyd[28477]: IMAP_Line_Read(): connection closed prematurely.
The director shows: Jul 11 21:10:14 cc-popmap7p dovecot: imap-login: proxy(jacintha): disconnecting 172.25.142.164
The backend server shows: Jul 11 21:10:14 cc-popmap2p dovecot: imap: user=<jacintha>, rip=10.142.0.162, pid=8629: Connection closed bytes=855/46109
Roundcube seems to handle the disconnects pretty badly, leaving a bunch of Apache processes chewing up CPU time.
-Brad