Turn off SharePoint Custom Error Messages

Here is how to turn off SharePoint custom error messages so you can see the “true” ASP.NET error message along with the callstack/stack trace. I would recommend only making the following changes in a development environment – not production. Navigate to the site directory, Backup web.config, Open web.config.

1. Switch Custom Errors off. Search for “customErrors” and set the value to “Off” instead of “On”. You can also set the value to “RemoteOnly” to troubleshoot a production issue.
2. Enable CallStack. Search for “CallStack” and set the value to “true” instead of “false”.
3. Save web.config.

That's all, now you should be able to view actual error message.