502 error - Bad gateway

This almost always is an application issue.  502  is a catch-all for  issues . Some of them listed below

• It's possible app is trying to write to a READ-ONLY file system.
• Or application is crashing due to memory leak or high memory consumption.
• Even bad routes in your routes.yaml or missing index html files could cause issues when the main site is looking for such files
• Tools like strace might help. Also do not forget to check the logs.
• Could be related to DB or other services like NFS being down. So check those too.