I don't know the cause of this, there are actually lots of possible causes.
While I do have demo.mojoportal.com running under .NET 4 with no troubles, I'm beginning to think it is not a good idea to encourage people to use .NET 4 hosting just yet. There may be some problems in some server configurations since we still really are compiled for 3.5 .NET. Maybe it will go smoother once we have packages compiled for 4.0 .NET. I am going to try sometime in the next few weeks to produce a build for 4.0, but I have to make sure we can still build also for 3.5 for a long time to come, so I have to be very careful when I do this. For now it might be best to stick with 3.5 hosting and wait until there is more proven success under 4.0.