When I google that error I see others are having it but I don't see answers other than what I have already suggested.
Is it a root level site or running below another site?
You might try using the Web.net4.mediumtrust.config as I would expect you are hosted in medium trust.
This release of mojoPortal was compiled against 3.5 .NET, maybe discountasp is only really supporting things compiled against 4.0.
In the future we will make packages compiled for 4.0.
demo.mojoportal.com is running fine under 4.0 in full trust using the Web.net4.config file.
Wish I could be of more help,