I don't know much about Allgäu, so I don't know if they have a history of acquiring rare species, but I feel tempted to believe it might be a Livingstone's turaco (or one of the other Knysna-like species). Zootierliste does state it's signed, but it wouldn't be the first time a zoo - especially one of the small, privately-owned zoos - signed one of their own animals wrong.