upon startup i get this error:

[17:13:39] [main/INFO] [GradleStart]: version: 1.7
[17:13:39] [main/INFO] [GradleStart]: tweakClass: cpw.mods.fml.common.launcher.FMLTweaker
[17:13:39] [main/INFO] [GradleStart]: username: sigurd4
[17:13:39] [main/INFO] [GradleStart]: Extra: []
[17:13:39] [main/INFO] [GradleStart]: Running with arguments: [--userProperties, {}, --assetsDir, C:/Users/Sigurd/.gradle/caches/minecraft/assets, --assetIndex, 1.7.10, --accessToken, {REDACTED}, --version, 1.7, --tweakClass, cpw.mods.fml.common.launcher.FMLTweaker, --username, sigurd4]
[17:13:39] [main/INFO] [LaunchWrapper]: Loading tweak class name cpw.mods.fml.common.launcher.FMLTweaker
[17:13:39] [main/INFO] [LaunchWrapper]: Using primary tweak class name cpw.mods.fml.common.launcher.FMLTweaker
[17:13:39] [main/INFO] [LaunchWrapper]: Calling tweak class cpw.mods.fml.common.launcher.FMLTweaker
[17:13:39] [main/INFO] [FML]: Forge Mod Loader version 7.10.18.1180 for Minecraft 1.7.10 loading
[17:13:39] [main/INFO] [FML]: Java is Java HotSpot(TM) 64-Bit Server VM, version 1.8.0_20, running on Windows 7:amd64:6.1, installed at C:\Program Files\Java\jre1.8.0_20
[17:13:39] [main/INFO] [FML]: Managed to load a deobfuscated Minecraft name- we are in a deobfuscated environment. Skipping runtime deobfuscation
[17:13:39] [main/WARN] [FML]: The coremod codechicken.core.launch.CodeChickenCorePlugin does not have a MCVersion annotation, it may cause issues with this version of Minecraft
[17:13:40] [main/WARN] [FML]: The coremod codechicken.nei.asm.NEICorePlugin does not have a MCVersion annotation, it may cause issues with this version of Minecraft
[17:13:40] [main/INFO] [LaunchWrapper]: Loading tweak class name cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker
[17:13:40] [main/INFO] [LaunchWrapper]: Loading tweak class name cpw.mods.fml.common.launcher.FMLDeobfTweaker
[17:13:40] [main/INFO] [LaunchWrapper]: Calling tweak class cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker
[17:13:40] [main/ERROR] [LaunchWrapper]: Unable to launch
java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(Unknown Source) ~[?:1.8.0_20]
at java.util.ArrayList$Itr.remove(Unknown Source) ~[?:1.8.0_20]
at net.minecraft.launchwrapper.Launch.launch(Launch.java:117) [launchwrapper-1.9.jar:?]
at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.9.jar:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_20]
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_20]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_20]
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_20]
at GradleStart.bounce(GradleStart.java:107) [start/:?]
at GradleStart.startClient(GradleStart.java:100) [start/:?]
at GradleStart.main(GradleStart.java:65) [start/:?]
Java HotSpot(TM) 64-Bit Server VM warning: Using incremental CMS is deprecated and will likely be removed in a future release

i dont understand what i did wrong. i undid all the changes i had done since last run, yet it doesnt work.

thats strange. i used java 8 previously, and that worked.