All my OCS clients today were receiving a message that they "Cannot Synchronize Address Book." This is denoted by a red exclamation point in the OCS client, or the yellow mark on the OCS client in the taskbar.
There are a number of reasons this might happen. In my case, it was because the service accounts I created for the OCS server passwords expired. I went into AD and reset the passwords and then checked to have them never expire. When I rebooted the server, the same problem was there. I also noticed in my security event log, that I was being hammered with Security Event 537 failures.
After some digging around, I found that there was one more place I needed to reset the password. In IIS, expand Application Pools. You'll notice that your LSGroupExpAppPool is stopped. Right click on the pool and go to properties, then to the identity tab. Make sure you reset the password here and then start the pool.
You can then either have your clients reboot and the issue will be resolved, or have them exit out of the application and go back in. At first, my users were presented with the login screen and of course that threw them off. However, some users just waited 10 minutes and their clients automatically logged back in.