IMAP: Can't copy mail to sent folder

[Beonix]

Hello,

I have been using Zimbra Open source edition for about half a year in a production environment, slowly migrating 250 users from old POP3 server to Zimbra IMAP.

Everything was going well, no problems for half a year.

OS is Ubuntu 8.04.4 LTS x64
Zimbra version is: 7.1.0
Running IMAP over SSL on 993, SMTP over SSL on 465.

2 weeks ago(last update made about 5 weeks ago) users started receiving "Cannot copy mail to Sent folder" when sending messages. The mail, however, is being sent, but it does not reach the "Sent" folder. I'm experiencing these problems whith Drafts folder as well.
Then, 10 seconds later it starts working again.

So, sometimes one can copy to the "Sent" folder, sometimes not.

Both Thunderbird and Outlook clients are affected.

I cannot think of any way to diagnose this problem.
I see no error messages in /var/log/*

Any ideas?
Thanks

[Beonix]

Just noticed that I had 200 in "Maximum allowed IMAP threads"
Changed that to 2000. Hope it clears the problem.