Hello, Gary.
We're happy to hear that you like our version of Cassini better than others.
While we are looking into the problem, you could try to copy aspnet_client folder into the root of your application, which we think you already did, but this time do not modify the web.config. See if that helps. If not, please check what URL exactly your control is requesting. To do that you could use a great HTTP tracing tool called
Fiddler. To intercept requests with Fiddler you will need to make sure you access your application using URL containing either machine name, like http://machinename:port/, or IP address - not localhost and 127.0.0.1. With the help of Fiddler you should see the which URL returned error code very easily. That will tell you and us what needs to be done to correct the problem.
Best regards,
UltiDev support.
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