@Solgryn Actually it turned out that something else was the problem, not other preprocessor scripts of Unity packages: The Spine Preferences Default Shader name that is stored in the Assets/Editor/SpineSettings.asset file was set to a Universal RP/Spine/.. shader which does not exist in your reproduction project, since the Spine URP Shaders UPM package is not added to the project. The bug was suddenly strangely fixed by opening the Edit - Preferences - Spine window, which automatically corrects the shader name when the asset is not found, which at first led us a bit astray.

Anyway, we just released a bugfix for this issue (for both 4.1 and 4.2-beta), using several fallbacks when the default shader set in SpineSettings can't be found. New unitypackages are available for download here as usual:
https://esotericsoftware.com/spine-unity-download
Please let us know if this resolves the issue on your end as well. Thanks for reporting!

@Solgryn Thanks for sending the reproduction package, we received everything and can now reproduce the issue. We have created an issue ticket here where you can subscribe to receive notifications upon any updates:
EsotericSoftware/spine-runtimes2310
We will let you know here on the forum once we have a bugfix ready. Thanks for reporting!

I checked the versio nand it says 4.1.x

Please note that this is not the package version. The package version is the second line in the mentioned version.txt file:

This Spine-Unity runtime works with data exported from Spine Editor version: 4.1.xx
Package version: spine-unity-4.1-2023-06-14.unitypackage

We have updated the packaging automation scripts so that the Package version: .. line is the first in the file, perhaps this helps with future releases going forward .

@Solgryn Thanks for providing the reproduction assets. Unfortunately we were unable to reproduce the issue by copying these assets over into a Unity project using the latest 4.1 spine-unity runtime. It was imported without any issues on both a Unity 2022.1.22 project and a 2020.3 project.

Could you please check whether you're using the latest version of the 4.1 spine-unity runtime? You can find the version number in the Assets/Spine/version.txt file if installed via the unitypackage, or in the Unity Package Manager if installed there.