Cannot login ***/wp-admin. Get return error 502

May I ask is this error comming from Cloudflare or your origin host / server?
Can you post a screenshot of it?
Does anything else appear in the Console (Developer Tools)?

Have you tried turning the debug mode on and re-check if anything appears at the debug.log file?

Is WordPress admin working over HTTPS connection, or better to ask is SSL defined in the wp-config.php as follows on the below article:

As @erictung already asked if WordPress was installed and working over HTTPS (home_url, blog_url both on or with www prefix?), but may I ask could it be due to some cookie settings in a Web browser (or by the defined COOKIE_DOMAIN with or without www prefix in wp-config.php maybe)?

Nevertheless, have you checked if you block any Cloudflare connections to your origin host / server?