A recent Windows Update File KB2756920 causes multiple problems on SharePoint 2010 running on Windows Server 2008 R2. If you install this patch on your SharePoint Search Server it will break SharePoint Search. You will receive an Internal server error exception error whenever you try to search on SharePoint 2010. Additionally, we noticed problems when retrieving and/or saving attachments on the portal. In SharePoint Central Administration you may receive a Health Analyzer warning that the Security Token Service is not available.
Fortunately, the fix is simple. Uninstall the patch KB2756920, reboot the server and verify that search and file uploads and download are now working. You can install a newer hotfix for 3.5 sp1 KB2637518 or install Windows Server 2008 R2 Sp1, which will not break search.