Hello Markus,
If one virtual key isn't working properly on a given manual, whereas the rest are working, then that's definitely a hardware issue of some kind. For example, it could be a fault in the key's sensor, or its wiring, or its MIDI encoder. I can't really help meaningfully with that, but other forum users might perhaps have relevant suggestions to help fix it.
Hauptwerk doesn't have the ability to ignore just one particular key in a manual, I'm afraid. It treats all keys equally. I can only suggest either fixing the key hardware itself, or avoiding using that particular MIDI keyboard entirely.