Keyboard adjustment problem no longer "n"

The tiny "n" does not work anymore. This is a software and / or configuration problem because:

no keyboard problem with the USB connection to a laptop -> no hardware problem!
The recently purchased keyboard has the same problems / no problem related to a PC / connected to a laptop -> no hardware problem, it must be a problem of software / settings!
Start a live session Ubuntu 16.04: no problem -> a problem of settings !!!

But I have no idea what I changed for settings that could cause the problem (besides, I produced the "n" in this dialog only by pressing the "n" until that several "n" are displayed, then removing the redundant ones).

Clues about what happened?

Nikolaus