I was experiencing a 502 error when trying to save on Stadia. There was no user-friendly message to explain the issue, but I believe I've tracked it down - I had too many saves. Due to crashes I was saving often and creating a new save each time, and I had roughly 80 saves(!).
I don't know if it's a number of saves limit, or a total filesize limit - I also presume this is technically a Stadia limit I'm hitting - but there's no indication in game for the player to understand and fix the issue (by deleting saves). There probably needs to be some sort of progress bar to indicate the amount of save file space remaining.