If there is a System.Web.Extensions.dll in the big folder delete it, there should not be one as none is included in tha package. It should use the one in the GAC. I think the Mono 2.0.1 release must have some problems, they are very close to shipping 2.2, maybe that will be better, though at the moment I'm also getting a different error (but still related to scriptmanager) using Mono built from svn as mentioned in this thread.
Best,
Joe