Error 520 from time to time on all websites

All my WordPress websites are hosted on the same dedicated server and since 2/3 weeks ago they started to display the 520 Error. It’s impossible to replicate the error and download the HAR file because it doesn’t happen with a high frequency. I can open 100 pages without any problem and suddenly it is displayed the error. If I reload the page, it immediately works. As far as I experienced, it only works when I’m working the WordPress backend, editing the sites. I contacted Hostgator and they said it has to be Cloudflare to fix it.

As explained, I cannot replicate the issue so the best I could find in the error logs was this one:
[Tue Sep 07 07:14:03.830812 2021] [autoindex:error] [pid 13954:tid 47764501124864] [client] AH01276: Cannot serve directory /home/garajau/public_html/images/: No matching DirectoryIndex (index.php,index.php5,index.php4,index.php3,index.perl,,index.plx,index.ppl,index.cgi,index.jsp,,index.phtml,index.shtml,index.xhtml,index.html,index.htm,index.wml,Default.html,Default.htm,default.html,default.htm,home.html,home.htm,index.js) found, and server-generated directory index forbidden by Options directive, referer: []

Example of a website where this is happening:

Websites are using Cloudflare since 6/7 years ago and have the latest updates, including WP 5.8.1, PHP 7.4.23. (tried different versions but still got the error). Also made a database cleanup, optimized WP tables, post-cleanup, re-cored websites, cleaned all caches (wp rocket, browsers and cloudflare), changed SSL option from Strict to flexible, turned ‘always on’ feature off, but with no success.

I saw that Cloudflare fixed a Error 520 issue a few weeks ago but, in my case, I still have the error.

Thank you.

ticket number 2253860