Microsoft has acknowledged a problem with Exchange Server 2010 SP3 Update Rollup 8, which I first saw described on the German language site heise.de. Apparently there have been many more reports of problems.
The official announcement was tacked on the end of the Technet article covering the December Exchange releases:
An issue has been identified in the Exchange Server 2010 SP3 Update Rollup 8. The update has been recalled and is no longer available on the download center pending a new RU8 release. Customers should not proceed with deployments of this update until the new RU8 version is made available. Customers who have already started deployment of RU8 should rollback this update.
The issue impacts the ability of Outlook to connect to Exchange, thus we are taking the action to recall the RU8 to resolve this problem. We will deliver a revised RU8 package as soon as the issue can be isolated, corrected, and validated. We will publish further updates to this blog post regarding RU8.
This issue only impacts the Exchange Server 2010 SP3 RU8 update, the other updates remain valid and customers can continue with deployment of these packages.