Hey Alexandru & Ice13ill,
These error codes represent several different issues.
500 means an internal server error, which is not specific enough to determine a cause unless you check the logs of your app / server.
404 means a resource was requested but not found, most often since no request handler is configured for the route requested.
104 is an
App Engine error code used when concurrent requests are not configured on your app (threadsafe is false in app.yaml or appengine-web.xml)
Finally, ConcurrentModificationExceptioYou can read about how to avoid this in the documentationn is an error relating to the concurrent modification of a Datastore entity group too often in a given timeframe. You can read about how to avoid this in the documentation
[1],
[2].
If you could post the full stack traces / error logs, this would greatly increase anybody's ability to determine what was happening.
Now that the issues have resolved, it's possible that it may simply be beyond anyone to try to determine what's happening. Feel free to ask any questions you may have, I'll be happy to help.
Sincerely,
Nick
Cloud Platform Community Support