I had the same issue on Ubuntu 20.04, but could not understand why. I tried all the above options but they did not quite work for me. Signing packages seems a bit complex and unnecessary, so I am not quite sure if I did it right, but I managed to figure it out in the end and solved the problem.
What I realised was that this issue occurred only after performing a kernel/dist upgrade. If you manually compile any program on your system and then upgrade your kernel or OS then you get the same problem.
The solution was to install the virtualbox-dkms package. If you look at the installation packages on Synaptic Package Manager you will find loads. (I had virtualbox-6.1)
If you are not sure, just run the following command:
sudo apt install --reinstall virtualbox-dkms
It will first ask to uninstall the current version if it isn't the DKMS and ask to type -y to confirm. (Note: this will not delete your VMs)
I hope this works as an alternative solution if anyone is still scratching their heads with this.
