I had to load a save and check, Jaheira can not be buffed for me either. It says "Target must be an ally.", so it does sound like exactly the same thing.
So it is not tied to Nightsong or the fight at Last Light inn it seems like. I wonder what it could be. I had her join my party at once when she asked in the throne room after the boss fight. Minthara was sitting on the throne, out of the party at the time when Jaheira joined me so there was room in the party, and I sent Minthara to camp after her speech about Ketheric and Orin.
