Is there some way of finding a missing sharepoint user?

So I have administrator privileges to a sharepoint site. The site crashes on loading due to a user not found. I keep getting this error:

Microsoft.SharePoint.SPException: User cannot be found. 
at   Microsoft.SharePoint.SPUserCollection.get_Item(String loginName)

I have access to the server code to view it, but not for running purposes. I know it has something to do with a new migration of users.

Does anyone know if there's anyway using the sharepoint administrative functions to view which invalid user causes a site crash?