How to investigate WP 502 error for BackEnd?

Hello dear community, support guys!
I’ve faced with Backend login with 502 error with CloudFlare.
I read this article – https://webpop.io/cloudflare/502-bad-gateway-error/

All was done – Cache cleared, CloudFlare was activated to the Debug mode, Plugins folder was renamed.
Server error log is empty.
But result the same – site with Template (front-end) is working
To enter Back-end is not possible.
Any additional workaround about it?
Thanks.

Could you share your experience how to get access to BackEnd?

Thanks, but you didn’t read my message 🙁
I told before: “All was done – Cache cleared, CloudFlare was activated to the Debug mode, Plugins folder was renamed.
Server error log is empty.”

After that I’ve paused CloudFlare, but WP-admin got me 502 error.
This is not problem of CloudFlare coz service is disabled.
Any workaround ?

I’ve solved after Server changes
Domains > example.com > Apache & nginx Settings
fastcgi_buffers 8 16k;
fastcgi_buffer_size 32k;
proxy_buffer_size 128k;
proxy_buffers 4 256k;
proxy_busy_buffers_size 256k;