@trafcio The problem seems to be that your installation of the spine-unity runtime was not a clean version of the spine-unity-4.1-2023-04-05 package. Re-importing the same spine-unity-4.1-2023-04-05.unitypackage package shows changes in the IKConstraints.cs file in Spine/Runtime/spine-csharp/. After the package was imported the issue was immediately resolved.

While re-importing the same spine-unity-4.1-2023-04-05 package, you could consider updating to the latest version of the spine-unity package in one go thereafter to save you some potential other issues which have been fixed in the meantime.

@trafcio Thanks for sending the reproduction projects, we received everything and can now reproduce the issue. We will get back to you here on the forum as soon as we've figured out what's going wrong. Thanks for reporting and sorry for the troubles!

trafcio This error started occurring after updating Unity to 2021.3.22 and Spine to 4.1.

@trafcio Sorry to hear you're encountering problems!

Which exact version of the spine-unity runtime (name of the unitypackage, also listed in Assets/Spine/version.txt or in Package Manager) are you using?
Which version of the Spine Editor?

How exactly are you setting the Bone scale? Do you apply it via code each frame (e.g. after the animation is applied), or do you apply it only once (as your animations are not overriding bone scale via keys)?

Tried installing Spine on a fresh Windows 10 install (not my main PC).
Worked fine for me.

EDIT: Just did an in-place upgrade from 8.1 to 10. Spine didn't seem to break.
Still opens and exports. And generally, Windows 10: so far so good.

Do update us, @trafcio!