Hi Pharan,
Pardon me for not providing the details.
I'm using Unity 5.3.4f1. I imported an animation made using Spine 3.1.02, I'm using the latest Spine runtime for 3.2.00 from the github. The animation has linked mesh in it. After dragging the json, atlas and png the skeleton data fails to import and after I press "Attempt Reimport", I got the error message above. Do let me know if you need any other details, Thanks!