Not sure what is going on here, just started a new world. Can't reproduce with BackTools alone - will investigate further.

net.minecraft.class_151: Non [a-z0-9/._-] character in path of location: minecraft:net.minecraft.class_1739{minecraft:air}
	at net.minecraft.class_2960.method_45137(class_2960.java:252)
	at net.minecraft.class_2960.<init>(class_2960.java:47)
	at com.daniking.backtools.ConfigHandler.isItemEnabled(ConfigHandler.java:39)
	at com.daniking.backtools.HeldItemContext.tick(HeldItemContext.java:44)
	at net.minecraft.class_1657.handler$znn001$backtools$tick(class_1657.java:7387)
	at net.minecraft.class_1657.method_5773(class_1657.java)
	at net.minecraft.class_742.method_5773(class_742.java:73)
	at net.minecraft.class_746.method_5773(class_746.java:219)
	at net.minecraft.class_638.method_18646(class_638.java:272)
	at net.minecraft.class_1937.method_18472(class_1937.java:480)
	at net.minecraft.class_638.method_32124(class_638.java:255)
	at net.minecraft.class_5574.method_31791(class_5574.java:54)
	at net.minecraft.class_638.method_18116(class_638.java:251)
	at net.minecraft.class_310.method_1574(class_310.java:1901)
	at net.minecraft.class_310.method_1523(class_310.java:1181)
	at net.minecraft.class_310.method_1514(class_310.java:802)
	at net.minecraft.client.main.Main.main(Main.java:250)
	at net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider.launch(MinecraftGameProvider.java:470)
	at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:74)
	at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23)

I have a pretty good Idea of the problem.
What version of the mod are you using?

I (probably) fixed this in the latest release.
But it runs only on 1.20.3/4.

Could you please test this again in the latest version?

backtools-1.20.1-2

That version works fine, but I can't test it in the instance it's crashing in (again, backtools-1.20.1-2 works fine on its own). My best guess is that some mod is mixing in to one of the Minecraft classes listed in the crashlog in an incompatible way?

Ok, I ported the fix back just for you: unofficial 1.20.1

The game launched, but I got the same crash with a different item once I got into the game

I was actually going to try backporting that myself this weekend. That worked, thank you for the backport!

Are you shure this is the same crash and does nothing say about belts?
In this case I will have a deeper look into the problem.

OK, I got another one.
Could you please test if this one works? unofficial 1.20.1
If this doesn't work, then we have to filter out AirBlockItems specifically.

@DanikingRD May I ask: What was the reason for creating a new Identifer in isItemEnabled, line 39 in com.daniking.backtools.ConfigHandler instead of just using Registries.ITEM.getId(item)?
Seems like you were trying to solve a problem here, but I can't figure out what that was.

OK, I got another one. Could you please test if this one works? unofficial 1.20.1 If this doesn't work, then we have to filter out AirBlockItems specifically.

@DanikingRD May I ask: What was the reason for creating a new Identifer in isItemEnabled, line 39 in com.daniking.backtools.ConfigHandler instead of just using Registries.ITEM.getId(item)? Seems like you were trying to solve a problem here, but I can't figure out what that was.

Tbh i don't remember if it was supposed to fix something. I wanted to refactor some of the code but it never happened. If you can find a better way of handling the config load feel free to submit that.

Alright then, there is the PR

It was, in fact, not the same crash. Thank you for catching that.
1.20.1-4 seems to work fine with the default config.

Nice! Thanks for testing!
Depending on DanikingRD's answer, I will open an PR to get this fix into the official versions!