Found a cure to running Hyper-V Server 2012 as a virtual machine (nested virtualization) after searching for hours.
The key was to add hypervisor.cpuid.v0 to the .vmx settings file of the virtual machine.
Now I can continue on with my Hyper-V lab. Thanks Chris!
via Installing Hyper-V Server 2012 on ESXi 5 | Wahl Network.