My modpack is crashing intermittently with no consistent way to reproduce. I play about daily and the crash happens about twice a month. This is in single-player mode, always at the same location in the library, based on stack trace/exception info. It doesn't appear to crash Java as it returns to the Java launcher OK. Three sample crash reports attached, together with latest minecraft/Forge log.
Notes:

1. I haven't yet tried disabling Colored lighting in configs (assuming that's even possible).
2. This is occurring mostly during gameplay and not at startup or when loading a save file. One occurrence immediately after loading a save, but the save loaded successfully on retry.

latest.log

Exception stack:
java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
at java.util.ArrayList$Itr.next(Unknown Source)
at p455w0rdslib.api.client.shader.LightHandler.update(LightHandler.java:150)
at p455w0rdslib.asm.Hooks.enableColoredLighting(Hooks.java:26)
at net.minecraft.client.renderer.RenderGlobal.func_174982_a(RenderGlobal.java:1133)
at net.minecraft.client.renderer.RenderGlobal.func_174977_a(RenderGlobal.java:1125)
at net.minecraft.client.renderer.EntityRenderer.func_175068_a(EntityRenderer.java:1334)
at net.minecraft.client.renderer.EntityRenderer.func_78471_a(EntityRenderer.java:1259)
at net.minecraft.client.renderer.EntityRenderer.func_181560_a(EntityRenderer.java:1062)
at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:1119)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:398)
at net.minecraft.client.main.Main.main(SourceFile:123)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

crash-2019-10-08_17.53.25-client.txt
crash-2019-09-17_03.30.23-client.txt
crash-2019-10-06_08.47.31-client.txt

Above exception is confirmed to be using the default config file. I will play with the settings a bit (apart from disabling colored lights completely) and see if it reduces the crash rate. Game is crashing more frequently now, 3rd time this week.

Same issue.

Seems to occur when moving around fast or when doing heavy chunk loading
crash-2019-10-17_20.07.00-client.txt

Getting same crash.

https://pastebin.com/6G3PsxwM

I haven't had any issues the last few weeks playing, but today it has crashed twice. I set "colored light shader options" to false, I'll let you know if it stops the crashes.

Same crash for me as well

https://pastebin.com/UYS1jxGD

This is on a multiplayer server. At this point, any attempt to log in results in a crash.

I set B:EnableShaders=false in p455w0rdsLib.cfg and that seems to have stopped the crashes for me, so if you don't mind turning the system off entirely there's at least a workaround

Setting to false has also stopped the crashes for single player

should be resolved since I removed shaders