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Upgrade on Virtualbox 4.3.4 - fail to start virtual machines - VT-x error

I'm using virtualbox to run couple of my virtual machines inside of the Windows 7 64bit. Two days ago I have upgraded my running Virtualbox software to new version 4.3.4. After upgrade I was not able to start and run any of my virtual machines (Win7 32it, Xubuntu, KaliLinux, Mint) which works fine inside of previous virtualbox version.
After start I have received window with message telling that VT-x is disabled inside of the BIOS. The log file I have found inside one of the machines telling the same:
00:00:01.008134 VMSetError: D:\tinderbox\win-4.3\src\VBox\VMM\VMMR3\HM.cpp(531) int __cdecl HMR3Init(struct VM *); rc=VERR_VMX_MSR_SMX_VMXON_DISABLED
00:00:01.008163 VMSetError: VT-x is disabled in the BIOS for Safer-Mode/Trusted Extensions.
00:00:01.010176 ERROR [COM]: aRC=E_FAIL (0x80004005) aIID={8ab7c520-2442-4b66-8d74-4ff1e195d2b6} aComponent={Console} aText={VT-x is disabled in the BIOS for Safer-Mode/Trusted Extensions. (VERR_VMX_MSR_SMX_VMXON_DISABLED)}, preserve=false
00:00:01.065228 Power up failed (vrc=VERR_VMX_MSR_SMX_VMXON_DISABLED, rc=E_FAIL (0X80004005))
Some guides recommends to enable virtualization support inside of PC BIOS, but as I mentioned, before upgrade everything work fine. Anyway I've tried it, hovever I have not found required option (older BioS, Q8550 Intel CPU). 
Inside of Virtualbox the acceleration settings which is available through virtual machine Settings -> System -> Acceleration tab is grayed out and unaccessible.
Then I found the solution, which recommends disable VT-x, so run 
vboxmanage list vms
where in my case shows my VMs
and then 
vboxmanage modifyvm <name or uuid of vm> --longmode off
in my case for example
vboxmanage modifyvm Win7-32 --longmode off
Now I'm able at least start and run Win 7 - 32bit, KaliLinux (32bit debian) VB machines. Other machine does not run (MInt, Xubuntu) with critical error.
  1. Note1: Downgrade to VBox 4.3.2 does not help.
  2. Note2: Try edit VM_NAME.vbox file where replace <HardwareVirtEx enabled="true"/> for <HardwareVirtEx enabled="false"/>
  3. Note3: After a while after writing this post I have found an easy way and I did upgrade of my MotherBoard BIOS (5year old) to the latest one. This solve my issue and i'm able finally use and setup VT-x (Settings -> System -> Acceleration tab is accessible now). This allows me to run again 64bit guest OSs inside of my VirtualBox.