I often see this shortly after adding a new domain or new accounts with the IM feature active. It looks like this when I add a new buddy:

2009-02-05 15:53:39,565 INFO [btpool0-340] [name=demo2@test.com;mid=1917;ip=x.x.x.x;ua=ZimbraW ebClient - SAF3 (Mac)/5.0.11_GA_2695.UBUNTU8;] soap - IMSubscribeRequest
2009-02-05 15:53:39,565 WARN [pool-5-thread-1] [] im - Discarding IQ roster packet of remote user:
<iq type="set" id="820-56" from="demo2@test.com/zcs">
<query xmlns="jabber:iq:roster">
<item jid="demo1@test.com" name="" ask="subscribe" subscription="to"/>
</query>
</iq>

I *think* the problem might be the "/zcs" on the from= line, but I'm far from certain about that. I seem to recall that restarting mailboxd helped, but I'm loathe to do that on a production server during the day. I *thought* it was working with zmprov flushCache account, but apparently not. Anyone have any ideas?

(Currently on 5.0.11 GA on Ubuntu 8)