Monitoring Service plugin blocks message delivery to client

Our server is Openfire 3.6.4 on Ubuntu 8.04.3 x64, running in a XenServer 5.0.0 VM. Monitoring Service plugin is version 1.1.1. We have about 30 users. Openfire is authenticating against a Windows 2003 R2 Active Directory service.

If I enable the Monitoring Service plugin, things work fine. I can enable monitoring all conference rooms with no problem. When I turn on monitoring all one-to-one chats though, the chats are logged properly and all messages arrive in the logs, however some are not transmitted to the intended recipient. I can’t find a rhyme or reason (yet) to when and why the messages drop. They’re sent, arrive in the logs, then fail to arrive at their destination.

For now I’ve disabled the one-to-one chat logging, but we need to get it running in the near future. Anyone else had or seen this problem? I couldn’t find it in the forum, so I’m guessing I configured something weird. The server isn’t too heavily loaded (CPU usage stays under 10%), disk activity doesn’t seem too extreme either, and the network interface isn’t particularly busy. A puzzling issue.