502 without reasons

We are using Cloudflare Access with Google as auth provider.

Some users experience 502 pages when the web application is making POST requests. Performing the same action in a new fresh Incognito windows works.

We KNOW that our server is not the problem as without Cloudflare Access we do not have these issues.

It is a plain Django admin site we are protecting with Cloudflare Access.

Any ideas on the above? We are experiencing 502 for some users, and not for others, and also depending on what browser is used, and if in incognito or not. I understand that 502 should be returned if we really timeout, or something else happens, but the Host for sure does not error.