I upgraded virtualbox from 5.1.8 to 5.1.18, and there was just one small snag. The upgrade did not remove the DKMS modules from the 5.1.8 version. But the debian package manager did not update the kernel headers when it upgraded the kernel.
So, upon manually upgrading the kernel headers, the system tried to rebuild the modules from virtualbox 5.1.8, which produced errors, because 5.1.18 had already been installed over the top of 5.1.8.
I ran vboxconfig to build the modules for 5.1.18, and it worked perfectly. I don't know if the 5.1.8 modules still exist after running vboxconfig. They aren't in /usr/src. So, maybe if you could make upgrades update the Debian DKMS virtualbox modules people wouldn't get the make error I got.
I am really impressed with how trouble-free virtualbox is. It's less hassle than VMware Workstation. The only reason I post this message is because it seems like the developers try to eliminate 'every' problem.
Thank you for the fine virtualization program. I was a VMware Workstation user for 10 years. But virtualbox is better!