at net.minecraft.entity.passive.AbstractDonkeyEntity.createDefaultAttributes(AbstractDonkeyEntity.java:39)

Is there a solution for this?

Okay so in your gradle.properties you need to change your yarn_version to 20w14a+build.4. This issue with the mappings will be fixed in a later version of yarn

Also this is more of a support issue. This is not related to fabric api at all.

oh so it's on a real client

EDIT: misread what you said liach

Update yarn to 20w14a+build.8 or later, which has this issue fixed.