Hi Harald,
There are no errors in the Console window. Above are screenshots of the SkeletonData and Atlas asset views in the inspector, and although the Atlas asset displays a message directing to "fix any compile errors", I don't see any.
I sent a sample project to the e-mail address you provided.
Thanks.
Okay, after playing around with the sample project I sent you I was able to get it working. If I delete all Spine files in the project before installing the new Spine-Unity runtime, and then import all the Spine animations, it continues to work after closing and re-opening Unity. I applied this to the main project and everything seems to be working now. Maybe there was a lingering file in the previous runtime that was interfering with the new runtime's atlas files? I don't know.