Please delete the SkinnedMeshAttachment.cs file. That was replaced some time ago.

It must have been a while since you updated.
I'll add this to the update notes. Thanks for reporting.

Just thought I'd make an extra note, there are errors being caused by your example C# Scripts as well, deleting them fixes this though.

That would happen if they were leftover files from the last time you imported spine-unity. Probably happens when you upgrade from 2.x to 3.0. Some example scripts were removed since then, though typically, you don't import the example scripts when you're updating or at all in your working project.