virtualbox.org

I obviously meant a link from a website that you already tested or something like that.

So, after a lottttt of tests I found out what the problem is. The vt -x / amd -v must be off. If I disable that option and try with a x86 host, the machine works

The problem is that option must be enabled in order to run x64 hosts, so every time I try to launch one 64 bits machine, the problem arises.

If I do uname -m I get the output: x86_64
This seems correct.

Do you have any idea how can I run x64 hosts on this? Bear in mind that I reinstalled the machine with the latest Yosemite version, and my Virtualbox version is 4.3.20.

Does anyone have an idea about this? There's seem to be a bug in my car, he can't cross all the roads

that is not your problem. 32bit guests run just fine with VT-x enabled and 64bit guests require it be enabled.

I'm getting the feeling you are trying to run VirtualBox inside a Yosemite host running VMWare Fusion or some other hypervisor.

Seeing you still haven't attached a log for any guest and not much useful info is forthcoming I will let someone else take over.

VBox.log wrote: 00:00:36.497176 ERROR [COM]: aRC=VBOX_E_INVALID_VM_STATE (0x80bb0002) aIID={8ab7c520-2442-4b66-8d74-4ff1e195d2b6} aComponent={Console} aText={Invalid machine state GuruMeditation when checking if the guest entered the ACPI mode)}, preserve=false
00:00:38.089888 Console::powerDown(): A request to power off the VM has been issued (mMachineState=Stopping, InUninit=0)

It does look like an interaction with some other software which is (or was) using VT-x, perhaps incorrectly. That could be other VM software, system level debuggers, AV etc. Only the OP can know what other software is present.

The OP did have VMWare Fusion installed but said he uninstalled it. There aren't many AV products for Macs and I doubt they would be using VT-x. As far as system level debuggers, it doesn't strike me as something the OP would use. Beats me.

FWIW, I have VMWare Fusion installed along with VirtualBox and as long as they are not running at the same time they don't interfere with each other.