Pale Oak Tree generation seems to fail ~80% of the time (estimate). Here is the stacktrace:

[15:08:25] [Server thread/WARN]: java.lang.UnsupportedOperationException: Not supported yet. [15:08:25] [Server thread/WARN]: at org.bukkit.craftbukkit.v1_21_R3.util.DummyGeneratorAccess.a(DummyGeneratorAccess.java:58) [15:08:25] [Server thread/WARN]: at net.minecraft.world.level.levelgen.feature.treedecorators.PaleMossDecorator.a(SourceFile:65) [15:08:25] [Server thread/WARN]: at java.base/java.util.Optional.ifPresent(Optional.java:178) [15:08:25] [Server thread/WARN]: at net.minecraft.world.level.levelgen.feature.treedecorators.PaleMossDecorator.a(SourceFile:65) [15:08:25] [Server thread/WARN]: at net.minecraft.world.level.levelgen.feature.WorldGenTrees.a(SourceFile:159) [15:08:25] [Server thread/WARN]: at java.base/java.lang.Iterable.forEach(Iterable.java:75) [15:08:25] [Server thread/WARN]: at net.minecraft.world.level.levelgen.feature.WorldGenTrees.a(SourceFile:159) [15:08:25] [Server thread/WARN]: at net.minecraft.world.level.levelgen.feature.WorldGenerator.a(SourceFile:168) [15:08:25] [Server thread/WARN]: at net.minecraft.world.level.levelgen.feature.WorldGenFeatureConfigured.a(SourceFile:28)

It seems to be an issue caused by the PaleMossDecorator.