Comment 10 for bug 217892

This problem was affecting me, also. It seems that gdm kills all clients immediately upon logging in, which effectively snuffs the x11vnc server, also. I have resolved this problem, however, by downloading the latest stable release (0.9.12) of x11vnc (http://www.karlrunge.com/x11vnc/) and building it from source. The latest version of x11vnc attempts to reopen after gdm kills all connected clients. The version currently in the repo does not.

It would seem that the gdm option "KillInitClients=false" is no longer supported, which would have been a viable workaround for this issue.