Since the error is apparently on "launch" it sounds like the browser should be showing an error screen. If it was a 500 error on postback of data at runtime the browser window would not be taken over by the error message. The key to getting stack trace information here lies in the web.config "customErrors" setting. You might just temporarily set this to "Off" so the error screen will display a whole stack trace. If it's easily recreatable you could try to launch then just copy/paste the error message w/ the stack trace. Here are the possible settings for "customErrors mode".
"On" Always display custom (friendly) messages.
"Off" Always display detailed ASP.NET error information
"RemoteOnly" Display custom (friendly) messages only to users not running on the local Web server. This setting is recommended for security purposes, so that you do not display application detail information to remote clients.
<customErrors mode="Off" />
Additionally, the error message will be logged in the Trace.axd ASP.NET trace. Since this is a rolling log, the log would need to be viewed pretty soon after the error occured.