[12:21:26] [main/WARN]: Missing data pack fabric/fabric-tool-attribute-api-v1
[12:21:26] [main/WARN]: Missing data pack fabric/expandedstorage
[12:21:26] [main/WARN]: Missing data pack fabric/simple_backpack

[12:21:26] [main/INFO]: Reloading ResourceManager: Default, craftingpp.zip, treecapitator-v2-5-4-1568017495.zip
[12:21:27] [Worker-Main-9/INFO]: Loaded 7 recipes

What version of Fabric loader and Fabric API are you using?

Fabric Loader : 0.8.7+build.201
Fabric API : 0.11.4+build.353-1.16

Is there anything else that appears in the log?

emm, nothing
I can see the SimpleBackpack mod in the client's log, but the server cannot see it. And cannot craft backpacks on the server. Sorry for my bad english.

This mod
https://www.curseforge.com/minecraft/mc-mods/simple-backpack-fabric

server side

[13:12:36] [main/INFO]: Reloading ResourceManager: Default
[13:12:37] [Worker-Main-10/INFO]: Loaded 7 recipes

client side

[13:15:12] [main/INFO]: Reloading ResourceManager: Default, Fabric Renderer API (v1), Fabric Networking Block Entity (v0), Fabric Key Bindings (v0), Mod Menu, Fabric Tool Attribute API (v1), Fabric Renderer - Indigo, Fabric Containers (v0), Fabric Object Builder API (v1), Fabric Biomes (v1), Roughly Enough Items, Fabric Events Interaction (v0), Fabric Crash Report Info (v1), Fabric API Base, Fabric Rendering (v0), Fabric Rendering (v1), Cloth Config v2, Fabric Rendering Data Attachment (v1), Auto Config v1 Updated, Fabric Resource Loader (v0), Fabric Textures (v0), Fabric Content Registries (v0), Fabric Tag Extensions (v0), Fabric Rendering Fluids (v1), Fabric Registry Sync (v0), Fabric BlockRenderLayer Registration (v1), SimpleBackpack, Fabric Mining Levels (v0), Fabric Renderer Registries (v1), Fabric Loot Tables (v1), Fabric Events Lifecycle (v0), Fabric API, Fabric Models (v0), Fabric Item Groups (v0), Fabric Object Builders (v0), Fabric Networking (v0)
[13:15:15] [main/INFO]: OpenAL initialized.

Hmm, are there any issues with the mod's datapack?

Maybe not? There are only three recipes in the datapack.
UPDATE: I tried other mods and had the same issues.

them starting disabled is indeed a problem, it looks, but might not be the whole problem.

.....uh,

MODMUSS.

there's some stuff that MixinResourcePackManager does client-only and some not. Time to split it up!

Alright, this is fixed, but I'm gonna make mod resource packs auto-enable before I do anything else.

#663

Looks like Modmuss' fix didn't work, then. Time for me to take a look.

I think I might have found the issue. ResourcePackManager isn't used directly on the server; a subclass of it is. I need to find that subclass and do the injection there for it to work.

Hm, it doesn't seem to be that...

WAIT A SECOND! MOD DATAPACKS ARE STARTING DEFAULT DISABLED!

Weirdly mod datapacks work in dev for me, but haven't tested a real server yet.