I've solved the problem, I think.
I did unscrew the back beam, then slid it under the warp and screwed it back in.
In the printed directions for warping the Jane loom, I needed the following information, which was tricky to find in a single place.
1. In the assembly of the new 16-shaft Jane, the Louet gentleman tells you that that the tails of the texsolv cords should face the weaver. That helps with the direction of the cords, which I couldn't tell from other pictures.
2. In a tips and tricks video, the same Louet gentleman points out that that the warp has to go over the front beam, and he mentions in passing that new weavers sometimes missed that. (I was one of them, and that comment was a clue for me.)
3. On the Louet site, there is a picture of the Jane loom that shows the direction of the ratchet on the warp beam, with red arrows.
I needed an image for both beams that shows which way the cords wrap around the cloth and warp beam, and then how the warp or cloth travels around the slanted piece (breast beam in front, and whatever the back slanted beam is called).
I have only ever woven on a Schacht Baby Wolf, and I'm a newish weaver at that, so I hope sharing this helps someone else.
Thanks, Cheryl Schwarz!!
Julia in Montana