Moonlight Lib 2.1.5 added an exception in BlockTypeRegistry which makes Vertical Slabs Compat Crash on launch if Jaden’s Nether Expansion since it tries to load items from blocks that shouldn’t be loaded as they’re compatibility with a mod not present (Gardens of the Death in the case of Yellow Nether Bricks as mentioned in the log)

This is on Forge-MC1.20.1.

Here's the log of a minimal build:
https://mclo.gs/TwiStJD

This was fixed by a recent moonlight lib update