The database user for the connection pool named |default|lo|, is not able to proxy to the schema named ET_DEV. This could be a configured restriction on the maximum number of database sessions or an authorization failure.
I have seen 503 errors before, but not one with default|lo. It might be because we imported the REST web services from another system and only noticed it now or it might have been other things, we are not sure what caused it. Other Oracle APEX workspaces and schemas didn't have this issue.
The fix for us was a 4-step process:
Export All Web Services
De-Register the schema from ORDS
Enable the schema again
Import All Web Services again
Hope it helps you when you have it.