HI There
We have a few users on both MAC and Windows (9.05) where communication happens only one way. In this example, Henry is active and communicating with David. David can send messages to Henry, but each message opens a new chat window. Henry is unable to send to David, and the errors show up on the first conversation.
David is online and can communicate with Kristopher, Mike, and Willy. When Kristopher invites David and Henry into a group chat, David cannot see messages typed by Henry.
They are on the same presence server (we actually shut one of our two servers off), and they have each rebooted and logged in again. Henry has tried a MAC client as well, with similar behavior. On the MAC client however, it suggests that "Awaiting Confirmation from LastName, David. Delivery of your message may be delayed or may not be received".

Any Ideas?
We do have TAC cases open, but nothing so far.