The forum was inaccessible via firefox for me last night. The whole evening, 502 everywhere.
Interestingly when I used a Tor browser it could get the front page reliably, and sometimes threads, though slowly. The slowness is the Tor system, so maybe that's a hint - speed dependent? - when a browser makes all the page element requests rapidly, the eevblog server fails and returns 502.
What intrigues me, is that the 502 error message is a specific thing. The eevblog maintainers know where that is, and have edited it in the past. Ref: the text "Thank you for your patience, Dave & gnif 24th March 2017: We are performing some ..."
Not that I know anything about large web server diagnosis, but if this was some electronics system I'd be trying the following:
* Make a change in what I thought was the '502 error message' text source. Both to check it's actually the right source, and to update that March message, which looks bad. Like no one could be bothered updating stuff.
* Search the server code for anything that can throw that 502 message. Stick some error logging messages in each one, or point each one at a unique 502 error text, to identify WHICH one is the cause.
Hopefully that would result in understanding the cause. Half way to a solution. Oh, and mentioning the identified cause in this thread, and what can be done about it, would be nice too.