Starting with Linux kernel 6.12, the KVM (Kernel-based Virtual Machine) module initializes virtualization features (such as VMX for Intel or SVM for AMD) automatically upon module load, rather than waiting for the first use. This change means that KVM claims control of CPU virtualization capabilities as soon as its kernel modules (kvm_intel or kvm_amd) are loaded, which can prevent other virtualization software like VirtualBox from starting VMs, as they require exclusive access to these features.
In order to correct this, in file /etc/default/grub add the line
GRUB_CMDLINE_LINUX_DEFAULT="kvm.enable_virt_at_load=0"If GRUB_CMDLINE_LINUX_DEFAULT exist in the file, add at the end of the line kvm.enable_virt_at_load=0 (with space before). An example:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash kvm.enable_virt_at_load=0"save and close /etc/default/grub file.
In Terminal run:
sudo update-grub