with 0.8.0 version of the plugin:

[02:11:59] [Server thread/INFO]: [MemoryLeakFix] Attempting to ForceLoad All Mixins and clear cache
[02:11:59] [Server thread/INFO]: [STDERR]: java.lang.NoSuchFieldException: NO_GROUP
[02:11:59] [Server thread/INFO]: [STDERR]: at java.base/java.lang.Class.getDeclaredField(Class.java:2610)
[02:11:59] [Server thread/INFO]: [STDERR]: at ca.fxco.memoryleakfix.MemoryLeakFix.forceLoadAllMixinsAndClearSpongePoweredCache(MemoryLeakFix.java:32)
[02:11:59] [Server thread/INFO]: [STDERR]: at net.minecraft.server.MinecraftServer.handler$bgo000$memoryleakfix$onFinishedLoadingWorlds(MinecraftServer.java:5793)
[02:11:59] [Server thread/INFO]: [STDERR]: at net.minecraft.server.MinecraftServer.method_3735(MinecraftServer.java:338)
[02:11:59] [Server thread/INFO]: [STDERR]: at net.minecraft.class_3176.method_3823(class_3176.java:169)
[02:11:59] [Server thread/INFO]: [STDERR]: at net.minecraft.server.MinecraftServer.method_29741(MinecraftServer.java:648)
[02:11:59] [Server thread/INFO]: [STDERR]: at net.minecraft.server.MinecraftServer.method_29739(MinecraftServer.java:264)
[02:11:59] [Server thread/INFO]: [STDERR]: at java.base/java.lang.Thread.run(Thread.java:833)

Full log: https://pastebin.com/raw/qjNwEZ2w

I'm also having this error

I'm aware and it will be fixed in the refactor.
You can downgrade your fabric loader version to temporarily fix this

This has been fixed in the betas