I would make sure you have .NET 3.5 SP1 installed. I would also look in the /bin folder as we used to include the 1.0 version of System.Web.Extensions.dll and it may be best to delete it from the bin if it exists. It should use the 3.5 version in the GAC.
If the problem persists, please try to produce it on the demo.mojoportal.com site and then tell me steps so I can see the problem.
Best,
Joe