There's no sign of corrosion so next causes are either in with some serious threadlock, or it's been cross threaded. However, seeing no signs of either on the thread sticking out the bottom, so there's something awry here.
Take TLP up on the offer: it gives him something to do and it'll probably stop this turning more expensive.
On closer inspection of the pics, I'm not so sure about the cross threading now: they could well look a little wrong. It might just be the pic, but the last 2 threads in the 'before' look flattened a bit. Not so easy to tell on the 'after' for some reason...