Xen VMs won’t start: “Cannot plug VIF”

I've been unable to solve this problem for a few weeks now, so i'll try posting it to my blog – maybe someone out there knows the answer.  Would very much appreciate a clue on this one. I've got a VM on a citrix xenserver 5.0.0 machine that won't start. it was running fine until it was rebooted on a Fri afternoon, a few weeks ago. Starting it from the (windows-only) xencenter GUI results in: "This VM cannot be started, …