At this point, @Stage4000, you should reach out to Support to see if they can offer any insight into why you’re seeing a 521 error that we cannot reproduce. Additionally, if you’re still seeing the unrelated API error on dash, let them know that & include a recent screenshot as I cannot reproduce that in your account.
When you contact Support, they’ll ask you if you’ve verified your web server is running normally (Quick fix 1) & perhaps ask if you’ve reviewed the error logs to find the error. Your health checks could in fact be introducing the 521 if your host is rate limiting.
Note that checking to see if other cloudflare zones with the same IP & nameservers have the same issue or not is not one of the quick fix ideas - it does not help answer the question of why is this site having issues.
OK, closing this thread in a few minutes as it’s not driving towards resolution. I’ll update it here with root cause if & when we hear from Support.