This is probably not the right forum.  However, I can offer insight as to catching the issue.  These are server-side issues.  The easiest way that I know of to catch them would be to Open visual studio.  Choose Tools -> Attach to Process.  Choose w3wp.exe from the list (make sure Show processes in all sessions is checked).  Now choose the menu item Debug -> Exceptions....    Check the Thrown box for Common Language Runtime Exceptions.  Click ok.

Assuming you have the appropriate pdb files in your dnn bin folder, the application should break on the line causing the exception.