520 status code returned

Getting 520 status code when trying to perform file upload. We upload files as smaller chunks. The issue occurs only randomly. Have tried below steps to troubleshoot the issue,
(i)allowlisting- cloudflare IPs in origin server and origin server IPs in cloudflare are allowlisted. (ii)Analysis on file size- tried uploading files <100MB that is the cloudflare domain upload size limit. (iii)Cleared cookies(so that it does not affect the headers size). (iv)Checking origin server logs(apache logs and symfony logs)- no logs were found. (v) Disabling cloudflare- issue was not recreated(issue occurs randomly). (vi)With different browsers(brave, chrome, firefox)- issue recreated irrespective of browsers.
Couldn’t find the route cause.

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