Ok finally made it to work, here's what was wrong:
Code
root@omvnas:~# apt-cache policy virtualbox
virtualbox:
Installed: (none)
Candidate: 5.2.18-dfsg-2
Version table:
5.2.18-dfsg-2 500
500 http://ftp.de.debian.org/debian testing/contrib amd64 Packages
5.2.18-dfsg-1~bpo9+1 500
100 http://httpredir.debian.org/debian stretch-backports/contrib amd64 Packages
as you can see - the first version it's trying to install is from testing, what? testing - I don't remember enabling it - and they I remembered - I installed restic a few months back and had enabled it for a bit just to install the latest version of restic, after that I must have forgotten to disable it. So all this time it was trying to install virtualbox from testing instead of backports. Anyway - disabled testing repo `/etc/apt/sources.list.d/testing.list`, apt-get install and behold it works. Thanks a lot to all for your help, as expected the issue was in me