you should not copy dlls from the .NET 4 version of mojoPortal into an installation of mojoPortal that uses 3.5 .NET
The problem on this thread is not the same problem you were having. The problem on this thread was caused by the .NET 4 version of mojoPortal has a reference to System.Web.MVC and the dll was not present, but the .NET 3.5 version of mojoPortal has no reference to System.Web.MVC, so it cannot be the same problem.
So you may have had some other error that looked similar to you but it was not the same and you solved it wrong by thinking it was the same and now caused yourself a different error.
You should remove the System.Web.MVC.dll from the bin folder and restore the mojoPortal.Web.dll from the mojoPortal package for .NET 3.5. Then if you get an error you need to find out the detail and maybe then we can help.
Hope it helps,