I've read about several problems regarding this update. A couple of weeks ago, I installed the Windows Updates for august. At least, that was the plan. After several hours, the installation progress was still 0%. So I canceled the installation, restarted the server and gave it another try. Same result. Determined it was UR2 for Exchange 2010 SP3 causing the problem. Instead of installing it through Windows Update, I downloaded the update and tried a manual install in the evening.
Next morning, it was still 'Preparing for update'. So I cancelled it again, did some googling about this. Cleaning the Temp-folder could help. So tried it again. In the afternoon, something happened - Outlook lost connection with Exchange. Checked the server and found out all Exchange and related services had stopped and were set to 'Disabled'. At first, I tried to fix this by restarting the services, and got Exchange going again. The Setup screen for the update was gone, btw.
Quite uncomfortable situation, so I restored a System State from some days earlier. Took a couple of attempts before it succeeded, but that's another story. Anyway, everything is back to normal, with the exception of me hesitating to install this update?
Simon