We are seeing the same behavior in a slightly different environment. We are currently using Outlook 2003 with zco 5.0.10, and are packaging our upgrade to outlook 2007 using the advice here:
Outlook 2007 improvements in the February 2009 cumulative update - Zimbra :: Wiki
Thus, we end up using Outlook 2007 SP1, and including both outlook-specific hotfixes from sp2, while staying on zco 5.0.10. This results in the same "unknown" behavior that you all are describing. I will attempt the install without one or both of the outlook-specific patches, to see where the problem is introduced, and report on what I find.