Heyo!

So just tried your map, so good so far. Works in single player, everything good. Jump onto a server and BAM, the map is black. Not even rendering entities. I get the border, and myself. Nothing else.

Versions

MineCraft 1.15.2
JustMap 1.0.50

Entities, players radar and cave mapping are disabled by default on the servers and can be enabled only via special command from the server. And I checked mapping on the server, it works normal. Try the latest version of map.

That is understandable, this is also normal surface mapping though.