[Sorry for breaking this thread, but I am back from vacation and the nameserver for my mail is down for the whole weekend]
Benjamin, I cannot reproduce this bug; as emacs does not handle ISO_Level3_Shift, my only bet is that something is wrong with your modifiers. Can you please run $ xkbcomp :0 and send me the server-0.xkb file generated by this command? And what does xev display when you press ISO_Level3_Shift+q? -- Denis
