It was nothing sort of Ddos attack and forum have not seen it in years as what @OP is talking about the gateway error which sometimes happens due to network connection and site not being able to load and this is normal in many cases as many have opened up forum in normal manner.

There was one similar thread here where it was showing 400 error so you can see it's normal nothing sort of big and last for few seconds only and once you clear browser cache and cookies it might load up again.