It seems to be working even with the import error popping up. I can create the SkeletonData resource and assign the .skel and atlas. What confused me was that the imported files don't have the Spine icon like in the official documentation so I thought none of it is working together with the import error:
Can't import skeletons
Yeah, I see this error message as well and I have no idea where it is coming from. I also have no idea why our icon doesn't show up in Godot 4.x. We supposedly do what the module documentation tells us to do (and it works for 3.x)
I've opened an issue here:
EsotericSoftware/spine-runtimes2385
I have the same problem, i need help please!
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.
Godot 4.1 doesn't recognize any skeleton format, neither .spine-json nor .skel 
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.
I have the same problem with the .spine-json format 
It still doesn't work, but thank you very much for your time; you're a great person!
8 días más tarde
umbekannt Hey, if you're still around, can you share your OS and hardware specs in the very unlikely case that they are a contributing factor?
Here're mine:
Spine 4.1.23
OS: Windows 11 Home version 22H2
CPU: slightly downvolted i9-12900k
RAM: 32Gb DDR4
Drive/s where Godot/Spine reside: Samsung 980 PRO PCle 4.0 NVMe M.2
GPU: RTX 3070Ti /w latest Nvidia gaming drivers
10 días más tarde
I am using Godot 4.1.2 with C# support
When exporting as binary I still cannot seem to use the .skel file in Godot editor.
I've been having a similar problem and was trying to diagnosis it using this thread: https://esotericsoftware.com/forum/d/24751-spine-godot-c-support/19 only to realize it wasn't addressing this problem but just that stuff wouldn't work after importing in general, here the example projects seem to run fine despite this error so maybe it's ignorable for now? Running windows 10 with spine version 4.1.23, with premultiply alpha disabled on exporting, tried running godot on different drives too.
10 días más tarde
The weird directory path is probably from the build machine.
I do want to use Godot, but I'm reluctant while this issue persists. If importing already is so error prone, I shudder to think what might happen 6-12 months in production.
un mes más tarde
Unfortunately, this still happens in Godot Engine v4.2.stable.custom_build.46dc27791
Whatever this is, it must be on the runtimes implementation side. I tried renaming everything to snake_case in case this is an issue with Godot's naming convention since I've heard it can mix up capital and lowercase letters on Windows. Also tried different SSDs in case of weird permissions thing. Still no dice.
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.