Thanks @
csdf, @
IMH and @
AgEngineer for the bug report. That little search icon on the flight entry screen triggers a script running on the server-side which used to successfully scrape the flight details from Google Flights. Like all good screen scrapers it works just as long as it takes the target of the screen scraping to change their layout, which Google have, and rather than barfing quickly the script was perpetually spinning away and chewing up application pool resources which prevents the site from serving subsequent requests until IIS automatically spins up a new pool to take over. The 502 Bad Gateway responses indicate that - I front IIS with NGINX, and when NGINX is trying to proxy requests through to IIS and can't you get a 502. You are getting a response from the server then, so it's not your ISP or connectivity, just the server itself is saying "I'm broken".
Anyhow, as the search function wasn't working I've taken it away pending a return if and when I find another source to scrape or query for flight details.