Debugging SharePoint errors is  a pain.  You normally get the the  "An unexpected error has occurred" error message is shown.

Sometime there is nothing in the event log or trace log.

To enable detailed error messages in the browser do the following in web.config 

<SafeMode MaxControls="200" CallStack="false"… />  change to  <SafeMode MaxControls="200" CallStack="true" … />

The customError setting must be changed to "Off":     <customErrors mode="Off"/>

After these changes, the "An unexpected error has occurred" should no longer be shown and you should a standard ASP.NET error page.