I'm having a very similar problem after upgrading to Windows Server 2012 / IIS 8.
The exceptions are being added to the Windows event log. Open Event Viewer from the start menu, expand Windows > Application on the left and look for events in the "Web Event" category.
On our server the following change fixed the problem:
In IIS Manager, go to ".NET Error Pages", then on the right choose "Edit Feature Settings..." and change Mode to either On or Remote Only.