We using SSL Full Strict and our server on GCP VM instance, on the first week everything works fine and suddenly error 521. We have checking the server using telnet and curl and our server still up and responsive. Our server IP at 35.184.19.122 . We don’t know whats wrong with this configuration.
We also check the server log at httpd/error_log and the error only show this things script '/var/www/html/index.php' not found or unable to stat
and there are so plenty. I think this error not related with my problem.
Does we miss something? Please your help as soon as possible. Thank you.
Sorry you didn’t get a response before now, here are some details on the error,
A 521 error happens when Cloudflare is unable to make a TCP connection to your origin server. Review the suggestions in this Community Tip for Quick Fix Ideas.
I suspect something on the server may be blocking or rate limiting requests from Cloudflare IPs, are you able to check this and whitelist the IPs? Also, the linked tip has some other suggestions
