Sorry, missed that.
The lighting is a 32bit program, which means it can only utilize 2 or 4 gigs of memory. If you're using 50% of your memory, you're at the absolute limit of what a 32bit program can access, thus the memory allocation will fail. Not sure what you're doing that would require that much memory, but... you're going to have to change something in your map so it can be lit.