I've been seeing a lot of these errors lately on the forums, mostly when i try to access the unread page, though i got it just now while trying to make this post, too
Service Unavailable
Service is currently overloaded. Please try again later
I'm assuming you're seeing a lot of bot scraping traffic. Cloudflare has some options for automated bot detection that are available even to free users, but if you don't want to use cloudflare, there are also some open source options like anubis.
https://github.com/TecharoHQ/anubis. I haven't used anubis myself but i've heard decent things about it. Cloudflare is probably easier to set up though.