Using rv5a1 (latest version at the moment)
Crashes at Forge's step 4*

Minecraft Version: 1.12
AE2 Version: RV5 Alpha 1
Forge Version: 14.21.1.2426

OptiFine and FoamFix are present, if that's relevant

Disabled OptiFine: The error persists, so it isn't OptiFine's fault.