From Discord:

It seems to have some form of conflict with the "DiUI" mod that someone else has created. I have seperatedly contacted themp, in case they can come up with a solution. But I was wondering if you were going to be creating an updated version of appleskin? It currently works fine with 1.14.1 but this conflict is unfortunate.
https://paste.ee/p/swYji

It seems your mod is attempting to inject where the other mod overrides Due to how theirs affects the in-game GUI it is probably not a concern for you to find a way to work with their mod.

Might be possible to mark certain mixins as optional to avoid this sort of thing.