REI had a major rework recently-ish, causing the compat mixin to crash the client.
Edit: I'm actually not sure this is the case, it looks like method and mixin headers line up, it's probably something more nuanced.

AppleSkin: 2.1.3
REI: 6.0.279-alpha

crash-2021-10-08_16.27.12-client.txt

I should mention that I'm doing all of this in a development environment since that may be relevant. I have appleskin as a jar file in my run/mods folder, and REI as an external library.

Try with v2.2.0, that particular compat mixin has been removed.

Also, have you seen this section of the README in the Fabric branch?

https://github.com/squeek502/AppleSkin/tree/1.17-fabric#for-mod-developers

That method of including AppleSkin in your dev environment should be much less error-prone.

Try with v2.2.0, that particular compat mixin has been removed.

Thanks, I'll give it a test when I have time.

That method of including AppleSkin in your dev environment should be much less error-prone.

I could do that as well I suppose. I didn't look into this initially because I don't have any reason to work with the API.