virtualbox.org

Intel E5450/5110 processors are overwhelmed by linux virtual machines in multi-processor mode.
If I turn off io apic then it's okay.
I could repeat on two separate machines, one has HP DL380 G5 ( CPU Xeon E5450), in the other Dell PowerEdge 2950 ( CPU Xeon E5110).

HP DL 380G5 runs Debian 8.9 64 bit, Virtualbox 5.2.0
Guest1: Debian 7.11 64 bit
Guest2: Debian 8.9 runs correctly with enabled I/O APIC and 2 vCPU

I tested Guest1 in other host machine (Supermicro X10SLD-F/HF, Xeon E1240 v3) works perfectly.

PowerEdge 2950 runs Debian 7.9, Virtualbox 4.3.36
Guest1: Debian 7.11 64 bit
Guest2: Debian 6.0.10 64 bit

Xeon E5450 is Harpertown, 10 years old. Xeon 5110 (not E5110) is Woodcrest, even older. Neither of these is suitable for virtualization nowadays, because neither has nested paging. Xeon E3-1240 v3 is a Haswell, a completely different kettle of fish, with far better hardware virtualization support. The E5450 is still a nice CPU, but not for virtualization; its slightly newer successors (Nehalem, Westmere) are far better in that regard.

If you provided a VBox.log from the Harpertown and/or Woodcrest system it might be possible to say more, as it is all I can say is "don't do it".

Thanks for the exhaustive reply, this will be the solution.
I tested on several CPUs, these worked:
Xeon E5520
Xeon E5530

Yes, those are Nehalem EP processors, also pretty old by now but it's the "modern" generation of Intel CPUs.