After a kernel update and the "signal-event post-upgrade; signal-event reboot", even with the dkms module, the vboxdrv is not loaded because the module is not compiled with the new kernel, so you need to launch this command line in a root terminal.
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After a kernel update and the "signal-event post-upgrade; signal-event reboot", the vboxdrv need to be compiled with the new kernel, a script is launched at the boot time to test if the compilation is needed.
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If you want to launch manually the compilation of virtualbos module you do these command lines in a root terminal.