502 Bad gateway while logging to the WP admin panel

If not major, wouldn’t it be minor? The error indicates an issue with the origin server. There could be some changes made, affecting the site. Best, I suggest you to read the following Tutorial for the detailed fix:

This indicates the configuration issue at the host. You need to fix that first before onboarding to Cloudflare.

This ain’t the best option as suggested by the ExpertReply . The best one is Full Strict. Read the Tutorial