I will try it out. I guess, that's why you have the "Setup" in Spine. But Spine is not free and so it is easier to get Spriter-animations. Also the support for Spine is only the ground. No Mesh-deforming, no skin swapping, no on-the-fly change of items like changing a weapon with attachments ...
Thanks for the answer, I did forgot, how awkward the Spriter-concept itself is in some parts.
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first I tried to insert a bone, I guess, I had to do it manualy every keyframe in every animation. Maybe it is easier in an text-editor or something like that.
But it didn't worked. So in the next step I erased the Slash-Image from every keyframe in every animation. And now it loads.
I guess, the problem in Spriter is the mixing-up between bones and images. You can transform images and also make them disapear, they have there own keyframes and also can have alpha and so on. Back then in 2013, I guess, Esoteric noticed that it is unpracticle and create Spine. And Spriter later implemented bones as well.
Also without the "Setup" at the beginning, Spriter is very confusing and a mess with larger animations, because you can't look into every keyframe for every possible picture popping up or not.
Because of that, I guess, it is easier to convert a Spine-file to Spriter. But if you have Spine, you wouldn't do it. But the other way arround would be a benefit, I guess. But the problem here is Spriter itself, you could read the XML and JSON, but how you interprete the images flying arround somewhere, you would create a bone for every image ...
And maybe this would be a mess in Spiner later also. Because you could do "Animations" on one bone in Spine. Multiple Sprites flipping.
So thanks for the hint. Puh, so I have some characters and I guess, they are all messed-up the same way