Everytime I try to load a new chapter of a manga or a new tab of mangadex it says 'bad gateway' since the server update a few hours before. I have to refresh the tab once or twice for it to actually load correctly or it directly doesn't work.
It's most likely server overload, the update just finished and people are rushing back to their unfinished reads.. kinda like a heavy foot traffic, even commenting may take a brief moment to load sometimes.
Maybe MangaDex can limit the bandwich when the traffic is heavy? I think slower download is still better than many failed downloads. Just a suggestion.