4021 on plesk shared hosting, http 500 error with call to legacyajaxcallsasmxjsdebug

I was hoping someone may be able to assist. I have

Umbraco 4.0.2.1 installed in a Plesk shared environment (m6.net), .net 3.5 (with 3.5 web.config from Codeplex)

Everything appears to work ok except when trying to delete (content node,template,css,doc type etc) I get an HTTP 500 error with legacyAjaxCalls.asmx/jsdebug

The returned message is;

System.InvalidOperationException: js Web Service method name is not valid.
   at System.Web.Services.Protocols.HttpServerProtocol.Initialize()
   at System.Web.Services.Protocols.ServerProtocol.SetContext(Type type, HttpContext context, HttpRequest request, HttpResponse response)
   at System.Web.Services.Protocols.ServerProtocolFactory.Create(Type type, HttpContext context, HttpRequest request, HttpResponse response, Boolean& abortProcessing)

If i drop the /jsdebug from the url legacyAjaxCalls.asmx web service appears to respond correctly.

Any assistance greatly appreciated.

thanks,
Daniel