krammor Hmmm, unfortunately the older version of Unity doesn't work well on my Mac PC, so I wasn't able to check with the same version, but at least as long as I installed spine-unity 4.0 2022-09-26 in Unity 2022, it installed fine.
Was it a new Unity project you were trying to install? If you have not tried to install into a new Unity project, you may want to do so. If it works, there may be something wrong with your project.
