Web Parts not displaying after the July 2013 Windows Update
Following the application of Windows Updates in July 2013, web parts configured in SharePoint Designer where the XSLT has been modified (for example, data view web parts) may show the error Unable to display this Web Part…. I have received a few emails and calls on this already so I’m sure a few of you have experienced it as well.
Error: Unable to display this Web Part. To troubleshoot the problem, open this Web page in a Microsoft SharePoint Foundation-compatible HTML editor such as Microsoft SharePoint Designer. If the problem persists, contact your Web server administrator.
Of course always followed by a wonderful long Correlation ID!
The issue seems to have resulted from Microsoft Security Bulletin MS13-052. As a temporary workaround, the KB2844286 update can be removed and an IIS Reset must be performed to make it happy once again.
Hope this helps!!