After upgrading to Visual Studio 2017 I found that the start page of ASP.NET projects would popup the message like the following.

It had nothing to do with file or pool permissions, or what account your were debugging under, or what sort of ASP.NET project it was, or anything else I could discover. Strangely enough, after clicking OK the debugging actually started (without a UI), so luckily I could temporarily ignore the problem and continue developing my web services.

After a couple of weeks this problem became so irritating that I had to have a fresh look to defeat it. By luck, or a hunch, or desperation, or who knows what ... in solution explorer I right-clicked the index.htm file and selected "Browse With ..." and a small dialog appears. In the dialog I notice that the selected item in the Browsers list is Microsoft Edge (Default).

Changing the default to Internet Explorer solves the problem.

So here we are again ... despite my decades of wide IT experience, it takes me hours of puzzling and frustration spread out over many weeks to defeat a problem that has a trivial fix. Extensive web searches on this problem revealed no useful hints whatsoever.