Can you reset mintmenu completely (remove it from the panel, open a terminal, type "rm -rf ~/.linuxmint/mintMenu", type "mintmenu clean", reboot, add it to the panel again) and then see if you can reproduce the problem?
Can you reset mintmenu completely (remove it from the panel, open a terminal, type "rm -rf ~/.linuxmint/ mintMenu" , type "mintmenu clean", reboot, add it to the panel again) and then see if you can reproduce the problem?