At least one of my players is getting the error now, even though it doesn't do it for me anymore, do you know if I have to get them to delete the NWjs folder as well? I wonder if I have to rename the game so it has a different folder name, and doesn't use save files from the newer NWJS (.../appdata/local/gamename).
It'll still write to the nwjs folder I think, so yes, they'd have to delete/rename the directory as well. Honestly, because of the improvements that have been made in more recent versions of NW, you're better off picking up Steam4C2 instead of trying to use the buggy Greenworks plugin from Scirra. Steam4C2 has a lot more features, and is actually supported by its developer.