I downloaded the 5.9.8d version on twitch, set my ram up to 6 gigs and launched the pack. It opened the launcher but when I told it to play the game, it crashed.

Expected Behavior

It should launch like normal and continue to the mod initialization process

Current Behavior

It crashes and says "java.lang.RuntimeException: java.lang.RuntimeException: Patcher expecting non-empty class data file for net.minecraft.crash.CrashReport$1, but received empty, your vanilla jar may be corrupt.
at net.minecraftforge.fml.relauncher.CoreModManager$FMLPluginWrapper.injectIntoClassLoader(CoreModManager.java:168) ~[forge-1.12.2-14.23.4.2715.jar:?]
at net.minecraft.launchwrapper.Launch.launch(Launch.java:115) [launchwrapper-1.12.jar:?]
at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.12.jar:?]
Caused by: java.lang.RuntimeException: Patcher expecting non-empty class data file for net.minecraft.crash.CrashReport$1, but received empty, your vanilla jar may be corrupt.
at net.minecraftforge.fml.common.patcher.ClassPatchManager.applyPatch(ClassPatchManager.java:113) ~[forge-1.12.2-14.23.4.2715.jar:?]
at net.minecraftforge.fml.common.patcher.ClassPatchManager.getPatchedResource(ClassPatchManager.java:76) ~[forge-1.12.2-14.23.4.2715.jar:?]
at net.minecraftforge.fml.common.asm.transformers.deobf.FMLDeobfuscatingRemapper.getFieldType(FMLDeobfuscatingRemapper.java:219) ~[forge-1.12.2-14.23.4.2715.jar:?]
at net.minecraftforge.fml.common.asm.transformers.deobf.FMLDeobfuscatingRemapper.parseField(FMLDeobfuscatingRemapper.java:192) ~[forge-1.12.2-14.23.4.2715.jar:?]
at net.minecraftforge.fml.common.asm.transformers.deobf.FMLDeobfuscatingRemapper.setup(FMLDeobfuscatingRemapper.java:161) ~[forge-1.12.2-14.23.4.2715.jar:?]
at net.minecraftforge.fml.common.asm.FMLSanityChecker.injectData(FMLSanityChecker.java:187) ~[forge-1.12.2-14.23.4.2715.jar:?]
at net.minecraftforge.fml.relauncher.CoreModManager$FMLPluginWrapper.injectIntoClassLoader(CoreModManager.java:163) ~[forge-1.12.2-14.23.4.2715.jar:?]
... 2 more
[14:24:57] [main/INFO]: [java.lang.ThreadGroup:uncaughtException:1052]: net.minecraftforge.fml.relauncher.FMLSecurityManager$ExitTrappedException" in the log.

Possible Solution

Steps to Reproduce

1.Install pack
2.Run minecraft

Log

https://gist.github.com/beewhisperer65/6204ae7c9c1a809cb08922985793ef9e

Detailed Description

Verify your settings align with this. https://www.reddit.com/r/allthemods/comments/968r4k/how_to_fix_modpack_wont_load_fps/

If that doesn't help someone should be able to offer you other suggestions shortly.

To comment on this as well, I've heard of similar issues when the native launcher is used. So this issue may be tied to the version of java that minecraft is using. Looking in the log, I see java 1.8.0_51 which is quite old, so going through that picture and updating java would be a good thing to try.