navigateTo 404 not found

Hi!

I try to open a html webresource as dialog like this:

 Xrm.Navigation.navigateTo({
           pageType: "webresource",
           webresourceName: url,
           data: params,
       }, {
           target: 2,
           height: 550,
           width: 600,
           position: 1
       }).then(function success() {
           CloseDialogCallback();            
       }, function error() {
       });

The dialog box opens but looks like this:

If I copy the blured URL (above "Server Error") into the browser, I can navigate as wanted. What could be false?

thx in advance

©a-x-i