For me - not solved
The same problem here.
slackware-current, slamd64-current, kernel 18.104.22.168, 22.214.171.124, 126.96.36.199 - no success
Most solutions on the net lead to solving the problem with permissions, but maybe there is something I'm missing.
Checked /tmp/.vbox-xxx-ipc, /dev/vboxdrv - both are OK. Idea for anything else?
Or maybe there is some library in slackware, breaking virtualbox?
There is also an open ticket for this problem, but no comments yet.
P.S. Previous versions work without problem on any combination of slack/slamd and kernel version, the problem is only with 2.2.0(OSE)