Thanks Mike,
Yes, Windows Event Log shows multiple DataFlex 18.2 Web Application Server errors (one every few seconds) relating to "communication"

"A communication error occured. Remote end of the connection has been closed.
Session aborted and all pending i/o cancelled."

The blank browser page I reported eventually showed the webapp. So I tried again, and am still waiting for page to show after 20 minutes. I think it will eventually show.
Something horribly wrong here.