There are 3 setting changes you need to make to the SharePoint web.config to show the real error messages and call stack – rather than the out-of-the-box message that is often very vague and unhelpful for troubleshooting.

The other day I still couldn’t get it to show the real error message even though I have applied these settings. It turns out there are 2 different web.config:

• Under C:\inetpub\wwwroot\wss\VirtualDirectories\80:  this web.config applies to the content pages in SharePoint.
• Under [SharePointRoot]\TEMPLATE\LAYOUTS:  this web.config applies to the application pages in SharePoint, i.e. pages under the _layouts/ virtual parth.

The 3 setting changes you need to make are:

• <customErrors mode=”On” />  –  change this to “Off”
• <SafeMode MaxControls=”200″ CallStack=”false” …..>  –  change CallStack to “true”
• <compilation batch=”false” debug=”false” optimizeCompilations=”true”>  –  change debug to “true”