I checked the logs with that old project and it definitely had to do something with the virtual path.
The error message is thrown because the virtual path is registered incorrectly (a mistake in the path or the resolver type)
The virtual path should be registered like
Virtual Path: ~/mypath/*
Resource location: SitefinityWebApp (name of the assembly)
The application was not restarted after registering the virtual path (it is required to activate the virtual path) or the file that is loaded trough virtual path doesn`t have its built action set to embedded
resource (right click the file ->properties->Build action).
So I'm not sure what else this could be. You checked all the referenced to the template (.ascx and .js files) in your upgraded project?