520 Error on a specific ISP but the ISP's other users can access the site

I am unable to access the site using one ISP but when I switch to the other and refresh it loads with no problems. There isn’t a problem with the server itself as the other ISP or other people can access the site with no problems. And I am not sure if it’s the ISP’s fault in this case as the customer support on the ISP’s site has stated that they have no problems accessing the site on their end. My ticket ID is #2188467 and I have included 2 pictures of my problem in my request. They show the site not loading and loading after I change my wifi from one ISP to the other

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The 520 error most typically means your origin reset the TCP connection during the handshake. Given that it works on some ISPs but not others… that likely means your hosting provider is treating different Cloudflare IPs differently.

Check point 3 here:

And also check your error logs in point 1, as well.

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Well, interestingly, after I made this post I got into contact with my ISP again as they were asking if it was solved or not and by their recommendation I ended up resetting my modem which solved the issue like it was no big deal. I have no idea why this worked or why this was an issue to begin with but it do be like that sometimes. (Note: No, I did not mess with anything in the modem’s control panel before this happened.)

Hmmm sounds like rate-limiting, you can only have so many concurrent connections or re-tries before you’re blocked for 30 minutes up to several hours.

If you’re developing through their proxy, I’d advise stop.

Possibly not the answer but since others connected and you couldn’t (maybe this wasn’t always blocked).

Another thought would be that your server had some invalid firewall presets or host had a block somewhere on them. But first case has been an issue for many and no “warning” from CF.

While I only understand half of what you said, I’m pretty sure it wasn’t rate-limiting as it wasn’t several hours but nearly a week at this point. I’m not sure about the proxy stuff, all I do is do some stuff at the php and make sure it works. I’m completely lost on the rest of the message so anyways thank you for the help.

My apologies,

You understood enough! It definitely wasn’t a rate-limit on your IP then a week is far too long for any issue to resolve. Tops 1 day in my experience.

The only way your home ISP or router could cause a 520 would be if your web server is running on your home connection.

If your web server is not behind your home connection, then most likely all you have done is hidden the issue from yourself. A 520 error might appear intermittent if your connection hits different Cloudflare PoPs or servers - but if you continually hit the same server, the issue will be consistent if your origin server is rate limiting or blocking some of Cloudflare’s IPs. The real issue is your origin server is resetting TCP connections (most likely cause) and you need to check the things I previously mentioned.

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