java.lang.RuntimeException: Mixin transformation of net.minecraft.client.main.Main failed
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.getPostMixinClassByteArray(KnotClassDelegate.java:252)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.tryLoadClass(KnotClassDelegate.java:150)
at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:155)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider.launch(MinecraftGameProvider.java:606)
at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:77)
at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23)
Caused by: java.lang.NoSuchMethodError: net.fabricmc.loader.api.metadata.ModMetadata.getCustomElement(Ljava/lang/String;)Lcom/google/gson/JsonElement;
at com.chocohead.mm.Plugin.getMixins(Plugin.java:287)
at org.spongepowered.asm.mixin.transformer.PluginHandle.getMixins(PluginHandle.java:128)
at org.spongepowered.asm.mixin.transformer.MixinConfig.postInitialise(MixinConfig.java:796)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.prepareConfigs(MixinProcessor.java:568)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.select(MixinProcessor.java:462)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.checkSelect(MixinProcessor.java:438)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:290)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClass(MixinTransformer.java:234)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClassBytes(MixinTransformer.java:202)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.getPostMixinClassByteArray(KnotClassDelegate.java:247)
... 6 more
Is this all i have to tell?

OptiFabric 1.14 doesn't support the latest Fabric API and Loader.
I've forked it and updated the mod though, it now seems to work fine for me:
https://github.com/Sjouwer/OptiFabric-1.14.4-Updated/releases

@Chocohead
If you want I'd be willing to make a pull request for a 1.14 branch. I'm not quite sure though what the best way to do that is, since I basically only backported stuff, so might be better if I could discuss it first with you if possible.