Sandbox freeze when booting with more than 8 cpu

I’m running Sandbox 2.1 on VirtualBox 4.3.18.
If I assign more than 8 cores or more than 32 GB of ram, if freezes when booting.

Host1: Windows Server 2008 R2 Datacenter w/SP1
4x Intel Xeon E5-4640 (32 cores/64 threads) and 196 GB ram.

Host2: Windows 7, 2x Opteron 6272, 32 cores

Additional data here:

https://forums.virtualbox.org/viewtopic.php?f=3&t=64725&p=304765#p304765