Ive had the problem since the day I purchased to game haha, I didnt think much of it becuase my wife and i didnt play at the time so i was just deleting worlds when i got bored.
How has this issue not been resolved? Just like the comment above mine, I prefer not deleting the world my wife and i spent hours on. I saved that world to my cloud and tried signing in after deleting it and it worked... I've checked for corrupted data on the switch and nothing was found. Once again i prefer not having to delete my world just to play.
I resetted the client and the game and still no luck. I just keep getting error 504.