I'm not sure what happened but suddenly my voxelmap no longer shows the explored areas that I've been to. I've looked into my voxelmap.properties file and verified that there are files in the Cache folder for the server I'm on. It worked a few hours ago but then I went offline for a bit.
I did not update any software, hardware or drivers and am running the latest video updates.
You could try renaming the cache directory and starting over. Chances are the cache got corrupted somehow, usually from not closing properly, or maybe an error. I'm not sure there's a way to repair them, and if there is, it might not be easy. In your log, have noticed any messages from voxel?