Running the latest 1.12.2 forge 2854. Only these two mods and the Forgelin-1.8.4 dependency installed, it boots me back to the launcher within seconds of pressing play. Likely a mixin issue, because the suggested fix (renaming the jar to "_VanillaFix") solves the problem. Seems you're already aware of this issue but the changelog says please report so here I am.

The same happened with VanillaFix + Tinkers' Construct.

mixin 0-7-0-8 compatibility