i tried the newest fabric loader and the 0.14.25 version

java.lang.NoSuchMethodError: net.fabricmc.loader.FabricLoader.getGameProvider()Lnet/fabricmc/loader/game/GameProvider;
at me.modmuss50.optifabric.mod.OptifineSetup.getLaunchMinecraftJar(OptifineSetup.java:299)
at me.modmuss50.optifabric.mod.OptifineSetup.getMinecraftJar(OptifineSetup.java:277)
at me.modmuss50.optifabric.mod.OptifineSetup.getRuntime(OptifineSetup.java:84)
at me.modmuss50.optifabric.mod.OptifabricSetup.run(OptifabricSetup.java:41)
at com.chocohead.mm.Plugin$$Lambda$60/1589707549.accept(Unknown Source)
at java.util.ArrayList.forEach(ArrayList.java:1249)
at com.chocohead.mm.Plugin.getMixins(Plugin.java:340)
at org.spongepowered.asm.mixin.transformer.PluginHandle.getMixins(PluginHandle.java:128)
at org.spongepowered.asm.mixin.transformer.MixinConfig.postInitialise(MixinConfig.java:796)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.prepareConfigs(MixinProcessor.java:568)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.select(MixinProcessor.java:462)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.checkSelect(MixinProcessor.java:438)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:290)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClass(MixinTransformer.java:234)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClassBytes(MixinTransformer.java:202)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.getPostMixinClassByteArray(KnotClassDelegate.java:422)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.tryLoadClass(KnotClassDelegate.java:323)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.loadClass(KnotClassDelegate.java:218)
at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:119)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:348)
at net.fabricmc.loader.impl.util.DefaultLanguageAdapter.create(DefaultLanguageAdapter.java:50)
at net.fabricmc.loader.impl.entrypoint.EntrypointStorage$NewEntry.getOrCreate(EntrypointStorage.java:117)
at net.fabricmc.loader.impl.entrypoint.EntrypointContainerImpl.getEntrypoint(EntrypointContainerImpl.java:53)
at net.fabricmc.loader.impl.FabricLoaderImpl.invokeEntrypoints(FabricLoaderImpl.java:383)
at net.fabricmc.loader.impl.launch.knot.Knot.init(Knot.java:160)
at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:68)
at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23)

i fixed it by using fabric loader 0.10.8

i need to stay with 1.16.5 for a server

You could alternatively have updated OptiFabric. Why would you use such an old version in the first place?

I meant an old version of OptiFabric. The latest version of OptiFabric for Minecraft 1.16.5 is 1.13.16.

But now that you mention your Minecraft version, you could use the ViaFabricPlus mod on newer versions of Minecraft to play on that 1.16.5 server...