Is this a clean installation or do you have also some custom features packaged in it?

This error could happen in custom features that were compiled against older versions of mojoPortal.

Some things in the mojoPortal.Business.WebHelpers namespace were moved into the mojoPortal.Web assembly so if you get this error with a custom feature you need to re-compile your custom feature after adding a reference to mojoPortal.Web project in the palce where the error is happening.

The stack trace you posted of the error is not showing enough detail, it isn't pointing to code in any included features.

Or this error could happen if the file /bin/mojoPortal.Business.WebHelpers.dll is missing or the app pool user does not have read permission on the file.

Hope that helps,

Joe