umbekannt uff, then I dunno. Try resetting Spine export Pack Settings to Default. There's a button for it, then disable the Premultiply alpha checkbox. There's a note in docs that premultiply alpha is not supported. That's my last guess.

umbekannt You have .json instead of .spine-json. http://esotericsoftware.com/spine-godot#Exporting-for-Godot

Note: You should prefer binary skeleton exports over JSON exports, as they are smaller in size and faster to load. If you must use JSON, make sure the file extension is .spine-json instead of .json. If you have an existing project that still uses .json export files, you can use this Python script to convert it to the new .spine-json extension.

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And try one format at a time, delete contents in Godot, then reexport with correct file extension.

umbekannt Ignore the error for now, everything works both in the editor and in the build. Create SkeletonData Resource, then assign .skel or .spine-json and atlas to it. Add a SpineSprite node to scene and assign the newly created resource to it. The skeleton will show up and work as intended.

Hopefully, this can be resolved soon.