Hello @dperezmavro, I would use the same steps:
- Enable the debug log, and recreate your 504 situation.
- Once you have the debug log, it will show the backend requests that have failed, and what the response from your pod is.
With that information, you can get a clearer picture as to what requests worked, which requests failed, and you can play the "what is different between these two?" game 
99% of the time, 504's in kubernetes are due to networking issues.
If you are still encountering issues @dperezmavro, feel free to open a new thread here, and we can work on your issue directly.
Thanks!