Comment 53 for bug 769644

I am affected too on Lubuntu 14.04 and I can confirm that the temporary fix proposed by Bruce is working for me as well.

If you delete .config/openbox or .config/lxpanel, the problem still is persistent because the skeleton version of lubuntu-rc.xml that is copied over (from etc) seems to be outdated / not supporting the current LXDE version.