The error is 508: Exhausted all retries for key: %s HTTP 0 (HTTP 429 (HTTP/1.1 429 ))
Upon further investigation, I noticed that very few users were being affected by this very large number of errors. Then, I received this report from a player:
This player is repeatedly getting this error on one of their save files, and it appears to be specific to his key.
This error appears to be dependent on the key that was inputted, and it fires for attempts to both load and save player data: