Some pages throwing 521 intermittently

[hc1501]

I am using Zencart v1.5.4.
Server OS: Linux 4.4.0-1128-aws Database: MySQL 5.7.33-0ubuntu0.16.04.1
Server Date: 07/11/2022 22:57:20 Database Date: 07/11/2022 22:57:20
Server Up Time: 22:57:20 up 1:24, 0 users, load average: 0.03, 0.15, 0.09 HTTP Server: Apache
PHP Version: 5.6.40 (Zend: 2.6.0) PHP Memory Limit: 1024M PHP Safe Mode: Off
PHP File Uploads: On Max Size: 256M POST Max Size: 256M
Database Data Size: 2,934,418 kB Database Index Size: 24,423 kB

Many of my pages on website are throwing 521 error. I dont see any error in logs folder also related to this. Can someone please help to resolve it?

Attached snapshot below:

[lat9]

Noting that zc154 is woefully old ... you should upgrade to zc157d in preparation for the zc158 release later this year!

That HTTP 521 code (googling http code 521) returns

521: “Web server is down.” Error 521 is a Cloudflare-specific error message. It means that your web browser was able to successfully connect to Cloudflare, but Cloudflare was not able to connect to the origin web server.