Anyone know a fix? Happens during the linking process, map has lighting built, havent changed anything since last successful build, umbra volumes are set up correctly.  If you have any info I would appreciate it, as currently I can no longer test my map at all.

I don't know if your problem still exists, but I have the same problem and I managed to compile my map by removing some prefabs and patches from the map. I first tried to change the scaling of the umbra_volume but it didn't affect to the compiling or linking, so I don't think the problem is the volume. I'm not sure though.