Mod version: Fabric 1.21.1 - 1.3.6
Crashlog

Crash on clicking the backpack settings button

Should be fixed in 1.3.9.
Apparently fabric just doesn't like the name of a mixin.

I don't know how to fix it.
I found the cause to be my fix_moved_wrongly.ServerGamePacketListenerImplMixin fabric mixin, but removing the method or even disabling the entire mixin through a plugin does not fix the issue. It seems to cause a crash even without doing anything, just existing. Only thing that fixes it is to remove it from horseman mixin config, which is not really an option.
The funny thing is that HorseBuff does not cause this issue (when installed alogside Traveler's Backpack), even though it has basically identical mixin.

This is not the first time I have issue with it, and it's only on fabric. Forge/NeoForge works flawlessly.

alr, i'll just try not to press the backpack settings button accidentally

I found Horseman v1.3.6 has the same crash with Accessorify v1.4.1 on 1.21.1 Fabric v0.16.14

it's fixed indeed, thanks