With SIP 2.5.0 I get the following error:

Image Processor: Error executing the action Fit
System.InvalidOperationException: Fit Web Service method name is not valid. at System.Web.Services.Protocols.HttpServerProtocol.Initialize() at System.Web.Services.Protocols.ServerProtocolFactory.Create(Type type, HttpContext context, HttpRequest request, HttpResponse response, Boolean& abortProcessing)

When I upgrade to SIP 2.5.1 I get the same but then with SetMatte instead of Fit

I never got these errors before in SIP 2.5.0 with the exact same settings.
This one reproduces the error:
www.droomsites.com/vanstokkum/admin/add_test.asp?sipinfo=true&sipdebug=true

This one is ok:
www.droomsites.com/vanosch/admin/add_test.asp?sipinfo=true&sipdebug=true


Can anyone please help me out here, since it already took me about 6 hours to figure out what's going on...