Hi @FOAM
That’s good that the undo button is working. Your pen probably sends a ctrl+z signal to the operating system which is Krita’s undo default shortcut. As Krita does not communicate directly with the pen, you need to set the buttons to whatever key or mouse combo Krita understands.
Try setting the button to E and see what happens. Hopefully your tablet allows you to set a profile for Krita so this doesn’t mess with your other apps.