How to enable debugging in a SharePoint Site
Via Renaud Comte
If you want to get the real yellow ASP.Net page with the real error message, just update your web.config :
<SafeMode MaxControls="200" CallStack="false" ...>
< !-- Become-- >
<SafeMode MaxControls="200" CallStack="true" ...>
<customErrors mode="On" />
< !-- Become-- >
<customErrors mode="Off" />
<compilation batch="false" debug="false">
< !-- Become-- >
<compilation batch="true" debug="true">
Just make an IISRESET to be sure (or use "%systemroot%\sytem32\IISAPP.vbs /a YourSharePointApppool /r" to be faster ) and open your site