Is the character with the missing hands wearing the Tarnished Charm (Neck slot item, The one that gives you better death saves)?
Equipping that charm causes the character wearing it to have their arms disappear from the elbows down. It doesn't seem to actually affect them, but can potentially make some cutscenes kinda strange.
That was indeed the problem - thank you Niara. I would never had guessed, that a necklace could cause arms to vanish. And the charm is quite dark, so I have overlooked it, when checking the character.
And yes, the romance scene with Shadowheart was unintentionally funny