Okay, I'll try that out. I'm at work right now, gonna reply later at home.
edit: nope, still get the error. I downloaded first the unitypackage again, then the spine-runtimes-master.zip and replaced it in the assets, still the question marks.
Fixed now, Mitch updated the spine unitypackage so it's new as github ones. I always downloaded the one from esoteric forums.