if a mask is used in the skeleton, an error occurs!
there was no such problem before, it is difficult to say exactly with which version it started, a month ago and earlier - such a problem was not observed!
Now, if I completely remove masks from all skeletons of the project and export the project, then after importing into unity no errors occur
how to live without masks?

please check if skeletons with masks are exported correctly.
NullReferenceException: Object reference not set to an instance of an object
Spine.Unity.MeshGenerator.AddSubmesh (Spine.Unity.SubmeshInstruction instruction, System.Boolean updateTriangles) (at Assets/External/SpineAnimation/Spine/Runtime/spine-unity/Mesh Generation/MeshGenerator.cs:490)
Spine.Unity.SkeletonRenderer.LateUpdate () (at Assets/External/SpineAnimation/Spine/Runtime/spine-unity/Components/SkeletonRenderer.cs:429)
Spine.Unity.SkeletonAnimation.LateUpdate () (at Assets/External/SpineAnimation/Spine/Runtime/spine-unity/Components/SkeletonAnimation.cs:247)
p.s.Spine.Unity.MeshGenerator.AddSubmesh (Spine.Unity.SubmeshInstruction instruction, System.Boolean updateTriangles) (at Assets/External/SpineAnimation/Spine/Runtime/spine-unity/Mesh Generation/MeshGenerator.cs:490)
Spine.Unity.SkeletonRenderer.LateUpdate () (at Assets/External/SpineAnimation/Spine/Runtime/spine-unity/Components/SkeletonRenderer.cs:429)
Spine.Unity.SkeletonAnimation.LateUpdate () (at Assets/External/SpineAnimation/Spine/Runtime/spine-unity/Components/SkeletonAnimation.cs:247)
Spine Pro 4.0.51