Version of mod: 1.0.2
Version of forge: 1614
When crash happens: Immediately upon joining world
Crash report:
crash-2021-09-04_19.05.58-server.txt
However it's pretty useless. More information is given in the console, namely:
http://hastebin.com/ukugapifuy.kotlin
Hoping you can take a look at this.

I'm guessing you're missing the program arguments for the mixin:
--tweakClass org.spongepowered.asm.launch.MixinTweaker --mixin satchels.mixin.json
Funnily enough, I was just thinking about adding this to the readme yesterday but forgot.

Added it now.

The crash still happens. In the log, I see "Completely ignored arguments: [--mixin, satchels.mixin.json]". Any idea as to why it's ignoring it?

You can ignore that message, it's not an error.

Does the crash happen with no other mods in the dev environment instance? Also, can you confirm Reference map 'satchels.mixin.refmap.json' for satchels.mixin.json could not be read. If this is a development environment you can ignore this message shows up in the log? That would confirm the mixin environment is running at least.

Sorry for the late reply. No, I cannot find Reference map satchels.mixin.refmap.json for satchels.mixin.json could not be read. If this is a development environment you can ignore this message in the logs, but I can find Remapping refMap satchels.mixin.refmap.json using.. etc. So I think mixin loads just fine. I deleted all other mods from development environment and it loads without the crash, so I assume it was a mod incompatibility. Sadly can't say which mod though, I just deleted all 25+ of them without thinking ahead. I apologize again.

This is why you should always move files to the recycle bin first before permanently deleting them 😏
Just kidding, no worries. Glad it turned out to be just an incompatibility issue and not something more critical.