When developing for example Web Parts in SharePoint 2010 sometimes it might be very helpful to see the full error message.
To achieve this you have to find the web.config file (it should be located in C:\Inetpub\wwwroot\wss\VirtualDirectories\<Site Collection Host Header – if exists><Site Collection Port>).
Then you have to open it with Visual Studio or Notepad.
Search for the following texts and do the changes as they are presented below:
- CallStack=”false” and change it from false to true
- customErrors mode=”On” and change it to Off
- Debug=”false” and change it to true