Just a note to anyone who may be getting this error: Unknown World!
VoxelMap is one of my essential mods and I love it, but I'd been having a nagging issue with "Unknown world" errors in chat whenever I'd change worlds. I tried multiple server plugins as suggested in this thread to manage the world names, but no matter how many times I'd chosen the world in the map UI it never saved.
I just found out it's because I was using our world names with colons. Our worlds always follow a naming convention of "Destinos:WorldName" and I was using that convention in VoxelMap.
When I just on a whim, thought to try "Destonos_Worldname" it started working fine So if you're having this problem, try eliminiating :'s (and maybe other special characters?) from your names in VoxelMap.