I am running 6.09. I too get that message on clients on AC startup, but it does so whether AC is running on the server or not. I usually start AC on the server after each weekly reboot so that it can run backup. This problem just appeared one day months ago with no change in configuration. Initially AC also did not run backup but miraculously it worked again one day, and has been backing up ever since. When support remoted in to fix another problem, he did see the message but was too busy fixing the lab interface to work on it. He suggested upgrading to 6.10 to get rid of this problem. I did not have time to do it yet. From your post it seems the problem is still there.
