Also sorry to ask the obvious but the user your accessing with does have SIMS permissions yeah?
On the SharePoint machine go to
Right click on the SLG.ExceptionHandling.config file and select Open With | Notepad
edit the line that reads
so that it looks like this
Where slg\testslg is the domain\username that you were using when the error occurred
Save the changes to this file and then re-load the webpart on which the error occurred
You should now see the FULL error message