It only happens with my character, I have tried making a new one and that character can enter the DLCs.
So I have tried following all the related suggestions I have found in order fix it, and none seems to work.
I have tried leveling the character, dropping all items, dropping all quest items, sent my companions away, and generally just tried to modify everything about the character a bit, (added one point to each skill on level up, etc.).
I tried making the new character have the same perks, in case one of them were messing up, but the character could still enter the DLC.
I am not using any mods.
And no, dropping many hours of gameplay and starting over AGAIN, is not a solution.