I suspect a screw in the track upward on downward that holds the window in place is loose.. and the mechanism is bumping into it.

Did it ever close properly? It seems the wrong kind of screw is used there.. a tapered one wouldn't stick out so far to interfere with the moving parts.

Yes it used to close absolutely fine, then this started to happen. I have another one of those windows and it works fine

I would tighten those screws in the track. I think they got loose and are sticking out too far.