Thanks for the reproduction package email.
Unfortunately, the package is missing all .meta files. As a consequence, all references in Unity are broken.
Please send us the project again with the .meta files included.
Thanks for the new reproduction package.
I just found the cause of the problem (and must admit I could have already seen it in the screenshots you posted):
The assigned shader that did not work in your 2D Renderer setup was
Universal Render Pipeline/Spine/Skeleton Lit
but this is the normal (3D) version. The following 2D shader should be used instead:
Universal Render Pipeline/2D/Spine/Skeleton Lit
Note that all shaders for use with the 2D Renderer will be listed in Universal Render Pipeline/2D/Spine
going forward.