Is this your first time building for iOS and iPad?
That error seems too cryptic. It's possibly a shader thing but we don't have much to go on, especially since Spine's Skeleton and SkeletonLit shaders actually don't have much by way of custom code. Just uses stuff that Unity already provides.
We'll have to contact Unity about it and wait to hear from them.
Do you have any more information about your setup so we can send it to them? xcode version? unity version? etc...