Please bear with me as I just inherited the OCS environment. I have quite a few users on a Citrix farm that intermittently receive blank instant messages. They get the chat window pop-up, but no text. Please let me know if you have any tips for me. At this time, I don't know if it is just particular users having the problem. I also don't know if the users are on the same citrix server if that makes a difference.
OCS is not supported in a Citrix environment. I would move towards the web client for Communicator. This will give the IM'ing functionality since they cannot do anything else anyway.