X does not start, because VirtualBox Guest Additions are not adapted to the new kernel yet.
–> Use kernel 4.17.14 as fallback.
(This problem is in VirtualBox only. As hardware install kernel 4.18 works fine.)
Kernel 4.18.5 does not work with virtualbox either (VM)
The current version of the virtualbox-guest-... packages is 5.2.18-dfsg-1.
Most likely we have to wait for their next upgrade to run any of the 4.18... kernels with X in virtualbox.
(Text mode does work fine.)
Why are you looking for salvation in broken dfsg versions?
https://download.virtualbox.org/virtualbox/5.2.18/
@axt: auch mit dem Orginal bauen die additions nicht
This morning virtaulbox has been upgraded to version 5.2.18-dfsg-2 .
Now kernel 4.18.5 boots fine – no problems with the graphical environment anymore.