Moonlight Lib 2.1.5 added an exception in BlockTypeRegistry which makes Vertical Slabs Compat Crash on launch.
This is on Forge-MC1.20.1.

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

It seems to be an issue on Vertical Slabs Compat's end
that mod is trying to add vertical slabs to mod compat blocks in JNE which are registered but don't have block items unless that specific mod was loaded (in this case Yellow Nether Bricks only registers a block item if Gardens of The Dead is installed)

report it to Vertical Slabs Compat instead.

Understood thanks!