Related to ChaoticTrials/CaveStone#2
For some more information, look there.
tl;dr: Game launches with Forge 43.0.8 and JsonThings 0.6.0 but stops randomly with white screen.

To reproduce, I trimmed down my pack. My pack only contains the mod itself and this as thingpack:
thingpacks.zip

latest.log:
https://gist.github.com/MelanX/7e62249c117ae4e2118198150728de86

Yes, on the third try I was able to reproduce it with 43.1.3. I'll try tomorrow with a fresh install. Maybe some "old" data (which shouldn't be loaded if the mods aren't present to load the config) interferes it.

Err, when does the freeze happen?

In the debug log, the latest entry is [Render thread/DEBUG] [net.minecraftforge.fml.DeferredWorkQueue/LOADING]: Synchronous work queue completed in 132.5 ms. When only looking at the game window, it happens before the red background with white logo and the loading bar is loaded.

Have you tried with a newer forge version? I can't reproduce the issue in 43.1.3

Sorry for the delay. I now created a new profile in CurseForge launcher with only JsonThings and Forge 43.1.3. Here you can find the export for the pack.
jsonthings-test.zip

I was able to reproduce it on the 3. startup.

The fact it takes more than one startup smells horribly of race condition.

EDIT: What I mean is that it's enterely possible it won't happen in my computer... I'll have to figure it out "blind".

A thread dump when it's stuck would be awesome.

If there's anything I can do to help you/make finding the problem easier, tell me.

Sorry, needed to find out how to correctly generate it. Here you have it: https://gist.github.com/MelanX/4009ca7c1201d0804a608e8c1c1a4d2a

I have worked on a potential solution which will either fix all the issues, or cause even more deadlocks.
Hopefully the former.

I have marked it as alpha until it has gotten some testing. https://www.curseforge.com/minecraft/mc-mods/json-things/files/3965805

I tried to trigger it at least 10 times now. It didn't happen again. I'd say that it's fixed.

Awesome!