Need to find out what error is happening now. See Basic Troubleshooting.
Make sure if you use the mojoPortal package for .NET 4 that you have also configured your site to use .NET 4. .NET 4 cannot use the same application pool as any .NET 3.5 applications. When you install .NET 4, it does not change configuration on your site to use .NET 4, you ned to do that yourself, it should show a new .NET 4 Application pool and you should use it.
Hope it helps,
Joe