Notepad++'s shortcut mapper doesn't allow pressing a button to set it for an action, it has to be chosen from a list. But this list uses an English keyboard layout as a basis, so what keys should be chosen for a German keyboard?
Example: You want Ctrl++ for zoom in Ctrl+- for zoom out (without num pad). But + isn't even on the list, because it's a combined key on the English keyboard layout.
(This Q&A is intended to save you time trying every option, I did that for you.)