Hi, thanks for your replies.
Unfortunately no keys worked.
But I found why leafpad is opening.
In ~/.icewm/keys there is also the entry
key "Super+t" leafpadSo I commented this line and the
key "Alt+Ctrl+t" xtermworked as expected.
But I still don't understand why the "Super" key overwrite the "Alt-Ctrl" key-combination.
Has somebody else this experience in Icewm, or is this a problem of my installation or notebook?