I just upgraded to 5.3 from 5.0.

 

Immediately after doing so, I get the standard aspx ("Server Error" in red) within the editor window.  It just stays there for an instant, then disappears and the content is then displayed. 

 

It's so fast that the entire error message doesn't even have time to load before.  Every thing else works find so far.

 

Any ideas where to look?