This issue happens when KVM modules keep using VT-x even when you'r not using QEMU
Generally this is enough to make your virtualbox work
sudo modprobe -r kvm_intel kvm_amd kvm
But in some cases, you have to restart the virtualbox kernel modules like this:
sudo modprobe -r vboxnetadp vboxnetflt vboxdrv
sudo modprobe vboxdrv
sudo modprobe vboxnetflt
sudo modprobe vboxnetadp
This temporary frees the VT-x so VirtualBox can run VMs again.
Note: If you want to persist this, then create a small systemd service that runs these commands automatically at startup
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