You have 8GB for After Effects, and you've set it so that each background rendering process should take 4GB. After the foreground process takes its share (1.2x4GB=4.8GB), you have 3.2GB left for background rendering processes, which isn't enough for even one at your current settings.
Why are you allocating so much for each background process? 4GB per process is typical for 4K digital cinema.
See this page for information about memory performance in After Effects: http://adobe.ly/dC0MdR
You only have 12 GB RAM, and you're trying to assign 4 GB each to your remaining CPUs... THerefore you've left zero RAM for anything else on your system. I assume that's why it's not working. 12GB isn't really enough for MP. I would at least reduce the RAM allocation per CPU to 1 GB then work up from there to see where your sweet spot might be.
Thank you, guys... I allocated 2GB per background CPU and that did the trick. You're right, Todd, I don't need that much RAM. I'm going to buy more RAM and make my machine run a little faster. Thanks again!!!