Hybrid is a graphical user interface, it is single threaded.
In when using x264 it uses the x264 command line encoder, when using x265 it uses the x265 command line encoder.
Both have options to requires xy threads from the OS.
How those are distributed is decided by the OS.
There is no such thing as as using a whole processor.
Process priorities and threads assinged to an application is decided by the OS, so unless the tools you use runs with admin rights (which Hybrid doesn't) it can't influence which threads it or the tools it uses gets assigned.
Error code 0x0000007f stands for UNEXPECTED_KERNEL_MODE_TRAP, cause for this usually is:
Searching the net for Error Code 0x0000007f in Windows should show tons of additional infos.
If you think that this is just causes by the encoder requesting to many thread both x264 and x265 allow to limit the number of requested threads.
x265->Misc->Threading&Co->Pools/Threads->1/XXX
x264->Base->Threads->XXX
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