Run the page outside the debugger by hitting Ctrl-F5 (instead of F5) just to see if the error's something from the ASP.NET plumbing. If the failure is before your page's code, then a lot fo the times the debugger can't get very far. Also, you typically will see the exception and the call stack right there in the output of the page.
If it's not that (I can't quite tell from the description) or it doesn't help, then under the menu Debug->Exceptions you can control the debugger behavior when there's an exception. Find the item in the list that says "Common Language Runtime Exceptions" and check the "Thrown" checkbox. This tells the debugger to stop when there's an exception thrown.
-Brock
DevelopMentorhttp://staff.develop.com/ballen