The front vertical edge of the window is supposed to slide up and down inside a rubber-lined U-shaped channel as a guide.

I will guess that due to other failures, the window shifted rearwards and the front edge is now hung up on one of the tops of the U, rather than being settled in at the bottom of it.

So I'd focus on the front edge of the window, moving it in and out (towards the vehicle left and vehicle right), hoping it can get unstuck and fall into the U channel. At that point, the rear problem will be fixed as well.

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2001 X5 3.0i, 203k miles, AT, owned since 2014