Hallo fey42
I've also just upgraded from 16.04 to 18.04. There were more problems to work around than I remember in previous Ubuntu / Ubuntu-Mate upgrades - or that might just be my memory being selective.
Anyway, back to your imediate concern - Virtual Box.
I have just installed this and it works fine.
- Install Virtual Box from the software boutique.
- Install your "guest" operating system, then shut it down.
- In the Virtual Box main window select the menu "Help" and the item "Virtualbox Website..."
- From the Downloads section download the "Virtualbox Guest Additions" and install them directly.
Want to add a "shared" folder? This post should help you:
Does that help? 