If you're on 4.5.4, try this:
Code:
zmprov ms <servername> zimbraImapDisabledCapability IDLE
and then restart the server.
That turns off IMAP IDLE on the entire server, but will probably fix your problem. In the 5.0 release there will be a way to do this on a session-by-session basis rather than on the whole server -- see
bug 15331 for details.