Seeing the actual error behind ‘An unexpected error has occurred’ in Sharepoint 2010

Usually this happens if an unhandled exception occurs in the code.

Seeing how you are having problems configuring the web.config to allow the website to show you the error and stack trace remotely,

you could adding TRY and CATCH blocks around parts of the code where you suspect the exception could be thrown... Then on catch you can do a redirect to a generic simple error page where you can display the exception's message.

Edit: you can also try redirecting to a custom error page from the on error event in global.

If you would like to know what exact error description is, then you need to make following changes in your SharePoint web application web.config file.

1. CallStack="false" changed to CallStack="true"

2. <customErrors mode="On" /> changed to <customErrors mode="Off" />

Save it and do IISRESET from command line.

Open Page again then you will get exact error.