localhost hangs if WiFi is turned on...

[StreetWalker]

Running XAMPP 7.1.1 on Mac OS X Sierra:

I have a student with a really strange problem I haven't seen before - I have about 50 students running the same OS X system version and XAMPP version without issue, except for this one student:

localhost can only access local pages (in her htdocs folder) when her Macbook Pro's wireless (WiFi) is turned off.

Apache appears to start with no problem - no errors in the log. However as long as WiFi is turned on, accessing localhost in a web browser produces nothing - it appears to be just hanging there waiting. The moment she turns WiFi off on her Macbook Pro, and we refresh the page, localhost works properly. Tried it in Safari, Chrome, and Firefox, and her system shows the same behavior in all of them.

There is no other copy of Apache running; we have tried changing the listen and server ports to 8080; but nothing works and there appear to be no conflicts that I can find.

Another curious clue: If we try to access 127.0.0.1 while WiFi is on, the browser loads 404 error page from her internet providers Web Site - which is really strange. (127.0.0.1 otherwise works just like localhost does when WiFi is off)

Again, WiFi on, no localhost. Turn wifi off, localhost works fine.

Any ideas on what to check to solve this vexing problem?