I found out why.
Even though I remove_trait "Factionheir" 1 from Denethor and gave it to the new heir and even though he held the Heir Crown, Denethor still figured as heir in the family tree and whereever the game checked for it (I should perhaps have given him the Exheir trait). And as Denethor is married, he cannot enter a marriage alliance.
She could still not marry Aragorn though, I guess he figures as married from the start even though it is not evident in the family tree.













