Hey having the same issue. I am wondering if because I saved my maps locally if the players can't access on their end. The map image was white to them and my gridline and details were not present. Might need to save to dropbox or something like that. I am going to be testing some methods this weekend and will follow up!

This can be either from the map image being missing but still cached on the host's machine, or because the client needs to disable mod threading.