Usually means the last time you played the game it did not successfully upload the cloud data at the end of a session. Happens sometimes.
Then your online cloud data is "out of date" because your local save on your PC is newer.
At the next time you start said game it should ask you if you want to load the old cloud save or the newer local save. Choose the local one, play normally and the next time you end your game session it should update your cloud save files with the current one.

Do not choose" use cloud save" when presented with the option at the start of a game because it will overwrite your local save game and you might lose game progress you had since the last correct cloud save upload.

I have the same issue, but only with Don't Starve. If I uncheck the Sync with Steam Cloud , restart steam, the slider is back to Synch with Steam again. Klei has been no help. I think Steam is having issues writing to Don't Starve's system.ini file. It has a line in there for syncing with steam cloud that doesn't change no matter what. If I manually change it to disallow steam sync then when I restart steam it does stay inactive in the properties of the game. And then, and only then can I play DS.

If I start the game any other way, i cannot see any saves, or start any new game.

I know it's a little different story than the OP, but I felt related.

I've tried every trick posted online to get this to sync and nothing works. IF I play on my Steam Deck, I can see my saves from 6 years ago and play them. It syncs fine on the Deck, but it doesn't help on PC.

It seems to have self corrected itself. 🤷‍♀️

nice, i've been trying for 5 days now

Sorry to hear that.

none of this works for me

nice
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