virtualbox.org

On some rare occasions I found a Guest unable to start, you get the 'starting ...' little screen and then nothing, today by chance I had it again and accidently right clicked above one of the VM icons at the bottum and it continued as normal (before I killed the process a few times before it ran normal). Seems like it starts in a paused mode.

Host and guest?
You may need to file a ticket with bugtracker on this (link is below my post)