I tried re-exporting my Spine project, this time on my Windows PC. (last Export was on my Macbook)
After export, I went into Unreal to the content drawer and click "Import". I choose my new .json file.
I get this message:
"Couldn't load skeleton data and/or atlas. Please ensure the version of your exported data matches your runtime version."
This is odd, considering I read through the versioning docs, and double checked - I am using the most recent Spine Pro install and UE4 runtime, which are said to be compatible in the README of the runtime.