So... I think I broke it. I can't see this issue ANYWHERE else, but somehow I managed to get the mod to throw this crash report
crash-2018-05-06_13.06.39-client.txt. Looking through the code this is quite obviously thrown here. I don't quite know how this could be happening, but if you could take a look that would be greatly appreciated, thanks.

Forge Version: 14.23.3.2678
Minecraft Version: 1.12.2

Maybe it would be possible to get an insight into how this is happening by wrapping the IOE in the RuntimeException that's thrown so it prints info for that too?

Is this issue happening persistently? If yes could you please try launching with movingworld removed?
I dont think it will be as simple as wrapping the Exception as this looks like a pretty weird classloading issue

RE consistency: Yes. It happens every time I start the game, at the end of pre-init. I will test now sans movingworld.

crash report without movingworld - it's exactly the same

I wrapped the exception in 3.6.0 (available shortly for 1.12)

Ok, i'll see what i can do for you.

crash report Looks like it's not liking the class loader passed to ClassPath.from, which suggests that something is changing the classloader. Maybe the next step is to print out what that classloader is?

Please add this debug-mod to your mods-folder and send me the full log: https://media.forgecdn.net/files/2562/6/canondebug.jar

Ah, i see. Seems to be an incompatibility with my launcher and the way it's including natives in the classpath. Here's the latest.log

Good to know the method can't handle wildcard paths - yet it's not really my fault.
As this is the only known case so far you could fix your launcher to include the specific files. (or try updating the guava library)
Yet that method is going to go away in a future version anyways