Hello,
Error 502 means that the PHP service did not respond to your nginx, possibly because it was too busy. It’s way too general of an error to draw any meaningful conclusions. I would look around in the server logs for anything strange. Think access logs, nginx error logs, mysql error logs, etc.
Also, I would like to ask you the following:
1. Do you get the 502 error every time you follow the cron job link, or only some times?
2. Since you have enabled the WordPress debug log, do you see any other errors in the wp-contents/debug.log file?
3. Which coin adapter or adapters are you using? If you are using any RPC-API wallets, could it be that a connection is timing out due to some firewall?
4. Did you change anything around the time when this problem first occurred?
5. Go to the dashboard, and inspect all the debug info about cron jobs: Cron jobs last ran on, Cron jobs last runtime (sec), Cron jobs peak memory, Cron jobs memory delta. What do these read?