I'm currently running MultiMC 5, and Fabric Loader 0.12.11. When I change everything back to 1.18 and switch to OptiFine H3, it loads correctly (I keep OptiFabric to 1.12.9). However, when I switch to 1.18.1 and OptiFine H4, I get an error and it fails to load Minecraft with error code -1.

Here is the error code:

Failed to start Minecraft:
java.lang.reflect.InvocationTargetException
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:568)
at org.multimc.onesix.OneSixLauncher.launchWithMainClass(OneSixLauncher.java:210)
at org.multimc.onesix.OneSixLauncher.launch(OneSixLauncher.java:245)
at org.multimc.EntryPoint.listen(EntryPoint.java:143)
at org.multimc.EntryPoint.main(EntryPoint.java:34)
Caused by: java.lang.NoSuchMethodError: 'void net.minecraft.class_156.a()'
at net.minecraft.client.main.Main.(Main.java:43)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:568)
at net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider.launch(MinecraftGameProvider.java:602)
at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:77)
at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23)
... 8 more
Exiting with -1
Process exited with code -1.

There are no other mods in my folder, only OptiFine and OptiFabric.

Same error.

Failed to start Minecraft:
java.lang.reflect.InvocationTargetException
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at org.multimc.onesix.OneSixLauncher.launchWithMainClass(OneSixLauncher.java:210)
	at org.multimc.onesix.OneSixLauncher.launch(OneSixLauncher.java:245)
	at org.multimc.EntryPoint.listen(EntryPoint.java:143)
	at org.multimc.EntryPoint.main(EntryPoint.java:34)
Caused by: java.lang.NoSuchMethodError: 'void net.minecraft.class_156.method_39982()'
	at net.minecraft.client.main.Main.<clinit>(Main.java:43)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider.launch(MinecraftGameProvider.java:602)
	at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:77)
	at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23)
	... 8 more
Exiting with -1
Process exited with code -1.

Using OptiFine_1.18.1_HD_U_H4.jar
And optifabric-1.12.9.jar

Update: I updated to Fabric Loader 0.12.12, as well as H5 preview, and it works now!