For anyone who runs into this issue, it's caused by a log file (output_log.txt) in the game's save data location on your computer (save data locations for each system can be found here: https://www.pcgamingwiki.com/wiki/Graveyard_Keeper#Save_game_data_location). The file can get large enough to exceed the allotted space for the game's save data on the cloud, causing cloud sync failure. Just delete the file or move it elsewhere to fix the issue. Fortunately, it takes a while for the log file to get large enough to do this. in my case, I put close to 60 hours into the game before it happened to me.