If you are on a non-official server, make sure, in the ServerGrid.Json file that the MetaWorldURL, the AuthListURL and the MapImageURL default code is removed, they are links to the official map and will not work. They format should read "MetaWorldURL": "", as an example.
This fixed my Fog of War issue the first time I set up the servers. On the new big patch, no such luck, we all have black screens and can only see beds on ships. I just added the MapImagesCache folder as mentioned above and added our map files to it, it did not work for us. Would be great to find a solution to this.
