This is merely an error message, and as you found, both editor and exports work fine despite it.

I have spent 2 days figuring out where it is coming from to no avail. We aren't doing anything crazy in the atlas importer either.

So for now, you can simply ignore this error and carry on. If I find a solution, I'll update the issue and this thread.

Mario I'm currently experiencing the same issue, yet it's not ignorable - following the tutorial I'm not able to import either the .skel-json file, nor the .skel file for either method. The file icon is X'd out and I just get "Error importing 'res://Art/Output/skeleton.skel'." messages. I can't just proceed as it gives me errors if I try use the file. If I drag the skel file to the Skeleton file res, it says:
"No loader found for resource: res://Art/Output/skeleton.skel (expected type: )"
If I try load it through the path it says:
"Cannot load resource from path 'res://Art/Output/skeleton.skel'"
As a result, the skeleton file res just stays <empty> as godot just wont let me input the file at all to it. currently, there's no way to continue past this if the error occurs, as far as I can see.

T.Fly() Yeah. Odd thing is if I use the example ones made, opening the files run fine if I don't change anything (I still get errors when I initially open it the same as mentioned before) yet visually hitting play they move and visually function like in the tutorial video. However, if I try change or alter anything to do with the skel files, I can't move them and get the errors popup. It's odd.

I also can't import mine ".skel" file at first. After using "Skeleton Viewer" to check example files and mine, I may know why:

The Spine version of example files is 4.1.23, but my version is 3.8.99.

May be only specific version can be used, but I can't find a way to convert its version.

Ryke but I can't find a way to convert its version.

Make sure the runtime and editor version match by opening your Spine project file in a version of the editor that matches the one required by the runtime you are using.
https://esotericsoftware.com/spine-settings#Version
Then exporting the skel or json file from said editor version.
https://esotericsoftware.com/spine-export#Binary

If you have an outdated json you need to upgrade, (as it seems from what you are describing) you will first need to match your outdated json version with the editor, by downgrading the editor to 3.8.99, import the skel/json, then save a new Spine project, from there you can upgrade the editor to the version that matches the runtime and open the saved Spine project to export a new skel.
https://esotericsoftware.com/spine-import#Data
https://esotericsoftware.com/spine-versioning

Erika It seems I have to buy Spine, thanks for your help.

Ryke For testing purposes, you may also find test projects here in various versions: EsotericSoftware/spine-runtimestree/4.2-beta/examples
Just switch to a different branch to find exported examples matching that version.

If you want to import your own assets then yes, you will need to have your own Spine license to use the Spine runtimes as well as the Spine editor.

Mario Hi, I am affected by this issue so I went through the official Godot repository to try and find any open issues related to this. I didn't find anything that could specifically explain this behaviour. Did you maybe try and open an issue in the repository over there with a description of the problem? If not, that might speed up the process of fixing this problem.

T.Fly() Ah I see now that posting in this thread might have caused some confusion because of the title. I was referring to the icons not working for the spine related files in Godot. I can import the skeletons without much trouble, sorry for any confusion I may have caused.
Yes,I am using the custom Godot build provided by esotericsoftware.