I received the same error and did some debugging. In my case the error was caused by CassiniWebServer.exe being unable to find Cassini.dll. One way to fix this is run build.bat (described ReadMe.txt) to compile Cassini and CassiniWebServer and adds Cassini.dll to Global Assembly Cache.
If you installed Cassini in the default location, you could also just run:
gacutil /i c:\cassini\cassini.dll
Hope this helps.