You need to import SomberNight's and Emzy's private keys in to Kleopatra. It is giving you a valid signature but it does not know if the keys are correct because you haven't imported them like you have with ThomasV's key.

Grab their keys from here (https://github.com/spesmilo/electrum/tree/master/pubkeys) and import them in Kleopatra. Then try again.