A 503 error, much like the 508 error,
typically indicates that the account has reached
its LVE (Lightweight Virtual Environment) resource limits.

These limits are in place to ensure platform stability / fair use
and include restrictions on CPU, RAM, and Entry Processes.

When an account exceeds these limits, a 50x 'resource limit reached' error is displayed.

You can kill the processes from the CLI and set your app to stay within reasonable limits.