Thanks for the reply - I've updated Unity and reinstalled the spine package to try to address this, so fingers crossed that it'll work.
Unfortunately, I'm now seeing a lot of new errors when I try to run the project at all, again related to editor errors. Are there any gotchas around any of these that might result in errors?
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I'm very sure that the AttachmentLoader script exists in my unity-csharp folder (right under my assets directory), so I'm not sure what the trouble is with the bottom one.