This may sound like a silly question, but have you flagged virtualbox (via pax flags) to at least "em"?
Unfortunately, linux-grsec prevents starting VirtualBox without paxd installed. EDIT: I'm not the only one with this problem. You'll find several threads in the Arch forum.
Padlouk, no they did not bother to read. Simple human laziness. "Never assume malice for what can be explained by stupidity/ignorance/apathy." -- Nietzsche Mrk
Debian has been very solid for me. Did a dist-upgrade from wheezy to jessie in May, no problem whatsoever. Agree with you here.