In web.config, make sure the following section exists:
<scriptResourceHandler enableCompression=”false” enableCaching=”true” />
First, make sure it’s not there already since that will generate errors. This fixed the problem for me.
Second, several others have reported that this might do the trick:
Lookup /_vti_bin/sts in IIS (for your SharePoint site of course). The application pool for that site will be set to ‘SecurityTokenApplicationPool’. Change that setting to match the application pool used for your site.
I wish I could explain more about the problem, but I really cannot. But hey, if these fixes work… 🙂