Edge file download error Couldn't download - Something went wrong

Question

My web page has a link

href="https://...myapi.../files/get-user-role-file?fileID=etc..." target="_blank"

The web api call (get-user-role-file) returns a file with a mime type and a content disposition of inline. the code returns a HttpResponseMessage. here is some of the code

                ByteArrayContent content = new ByteArrayContent(_bytes);
                string dispositionType = "inline";
                if (_mimetype.IndexOf("octet-stream") > 0) dispositionType = "attachment";
                ContentDispositionHeaderValue contentDisposition = new ContentDispositionHeaderValue(dispositionType) {
                    FileName = _filename
                };

                MediaTypeHeaderValue contentType = new MediaTypeHeaderValue(_mimetype);
                response.Content = content;
                response.Content.Headers.ContentDisposition = contentDisposition;
                response.Content.Headers.ContentLength = _bytes.Length;
                response.Content.Headers.ContentType = contentType;

This works in chrome and safari and firefox. In Edge it works for me in VS debug or in InPrivate mode or when run locally from my machine in IIS. It also works if the file type is one that can be opened in the browser OR if I have Fiddler running. BUT it does not work, for example, with a Excel doc in Edge run normally off of the web server. It does work for everyone else I have asked so far.

I have tried updating Edge and clearing all history.

What happens is a new tab is opened with the file URL and the downloads modal opens with a file named get-user-role-file.json and a message says "Couldn't download - Something went wrong...". In the network tab of the dev tools the call returns OK 200 and no error but also no response (Failed to load response data: No resource with given identifier found).

It's a mystery.