You didn't mention what version of mojoPortal.
The 188.8.131.52 release of mojoPortal includes Web.config files for 4.0 .NET, Web.net4.config and Web.mediumtrust.net4.config, you probably need to use the Web.mediumtrust.net4.config, backup or rename the current Web.config and try renaming one of the net4 versions to Web.config
demo.mojoportal.com is running on .NET 4 but some others have reported problems getting it working.
The next release of mojoPortal will have separate packages for 3.5 and 4.0 .NET wheres the current release is really compiled for 3.5 but can work in 4.0 using one of the net4 config files.
Hope it helps,