Steps to reproduce: use fabric api 0.20.0 and higher, launch minecraft with optifabric and optifine.
When launching the game, it's crashing with the following output:

java.lang.RuntimeException: Not a ModResourcePack!
at net.fabricmc.fabric.impl.resource.loader.ModResourcePackCreator.method_14453(ModResourcePackCreator.java:64)
at net.minecraft.class_1066.handler$zch000$addBuiltinResourcePacks(class_1066.java:536)
at net.minecraft.class_1066.method_14453(class_1066.java:82)
at net.minecraft.class_3283.method_29212(class_3283.java:44)
at net.minecraft.class_3283.method_14445(class_3283.java:37)
at net.minecraft.class_310.(class_310.java:471)
at net.minecraft.client.main.Main.main(Main.java:177)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:566)
at net.fabricmc.loader.game.MinecraftGameProvider.launch(MinecraftGameProvider.java:192)
at net.fabricmc.loader.launch.knot.Knot.init(Knot.java:140)
at net.fabricmc.loader.launch.knot.KnotClient.main(KnotClient.java:26)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:566)
at org.multimc.onesix.OneSixLauncher.launchWithMainClass(OneSixLauncher.java:196)
at org.multimc.onesix.OneSixLauncher.launch(OneSixLauncher.java:231)
at org.multimc.EntryPoint.listen(EntryPoint.java:143)
at org.multimc.EntryPoint.main(EntryPoint.java:34)

This seems to be caused by a recent change in the resource loader of the fabric api: FabricMC/fabric@8072a3a
Edit: woops, actually this wasn't an issue caused by optifabric, so i'm closing this... sorry for the inconvenience!