Thread Dump: https://pastebin.com/7Wub4Cc7

Uninstalling Alex's Mobs fixes the issue.

"Worker-Main-9" daemon prio=4 Id=134 WAITING on java.util.concurrent.CompletableFuture$Signaller@31e3d40e
at [email protected]/jdk.internal.misc.Unsafe.park(Native Method)
- waiting on java.util.concurrent.CompletableFuture$Signaller@31e3d40e
at [email protected]/java.util.concurrent.locks.LockSupport.park(LockSupport.java:211)
at [email protected]/java.util.concurrent.CompletableFuture$Signaller.block(CompletableFuture.java:1864)
at [email protected]/java.util.concurrent.ForkJoinPool.compensatedBlock(ForkJoinPool.java:3449)
at [email protected]/java.util.concurrent.ForkJoinPool.managedBlock(ForkJoinPool.java:3432)
at [email protected]/java.util.concurrent.CompletableFuture.waitingGet(CompletableFuture.java:1898)
at [email protected]/java.util.concurrent.CompletableFuture.join(CompletableFuture.java:2117)
at TRANSFORMER/[email protected]/net.minecraft.server.level.ServerChunkCache.getChunkOffThread(ServerChunkCache.java:1170)
...

extended trace:
at TRANSFORMER/[email protected]/net.minecraft.server.level.ServerChunkCache.m_7587_(ServerChunkCache.java:1135)
at TRANSFORMER/[email protected]/net.minecraft.world.level.Level.m_6522_(Level.java:179)
at TRANSFORMER/[email protected]/net.minecraft.world.level.Level.m_6325_(Level.java:4049)
at TRANSFORMER/[email protected]/net.minecraft.world.level.Level.m_8055_(Level.java:4541)
at TRANSFORMER/[email protected]/net.minecraft.world.level.LevelReader.m_46855_(LevelReader.java:153)
at TRANSFORMER/[email protected]/net.minecraft.world.entity.Mob.m_6914_(Mob.java:806)
at TRANSFORMER/[email protected]/com.github.alexthe666.alexsmobs.entity.EntityElephant.canSpawnWithTraderHere(EntityElephant.java:996)
at TRANSFORMER/[email protected]/com.github.alexthe666.alexsmobs.event.ServerEvents.onEntityJoinWorld(ServerEvents.java:494)
at TRANSFORMER/[email protected]/com.github.alexthe666.alexsmobs.event.ServerEvents_onEntityJoinWorld_SpecialSpawn.invoke(.dynamic)
at MC-BOOTSTRAP/net.minecraftforge.eventbus/net.minecraftforge.eventbus.ASMEventHandler.invoke(ASMEventHandler.java:73)
at MC-BOOTSTRAP/net.minecraftforge.eventbus/net.minecraftforge.eventbus.EventBus$$Lambda$4792/0x0000000800e4caf8.invoke(Unknown Source)
at MC-BOOTSTRAP/net.minecraftforge.eventbus/net.minecraftforge.eventbus.EventBus.post(EventBus.java:315)
at MC-BOOTSTRAP/net.minecraftforge.eventbus/net.minecraftforge.eventbus.EventBus.post(EventBus.java:296)
at TRANSFORMER/[email protected]/net.minecraftforge.event.ForgeEventFactory.doSpecialSpawn(ForgeEventFactory.java:201)
at TRANSFORMER/[email protected]/net.minecraft.world.level.levelgen.structure.templatesystem.StructureTemplate.lambda$addEntitiesToWorld$5(StructureTemplate.java:423)
at TRANSFORMER/[email protected]/net.minecraft.world.level.levelgen.structure.templatesystem.StructureTemplate$$Lambda$51550/0x0000000804f89c80.accept(Unknown Source)
at [email protected]/java.util.Optional.ifPresent(Optional.java:178)
at TRANSFORMER/[email protected]/net.minecraft.world.level.levelgen.structure.templatesystem.StructureTemplate.addEntitiesToWorld(StructureTemplate.java:418)
at TRANSFORMER/[email protected]/net.minecraft.world.level.levelgen.structure.templatesystem.StructureTemplate.m_230328(StructureTemplate.java:337)
at TRANSFORMER/[email protected]/net.minecraft.world.level.levelgen.structure.pools.SinglePoolElement.m_213695(SinglePoolElement.java:127)
at TRANSFORMER/[email protected]/net.minecraft.world.level.levelgen.structure.PoolElementStructurePiece.m_226509_(PoolElementStructurePiece.java:89)
at TRANSFORMER/[email protected]/net.minecraft.world.level.levelgen.structure.PoolElementStructurePiece.m_213694_(PoolElementStructurePiece.java:85)
at TRANSFORMER/[email protected]/net.minecraft.world.level.levelgen.structure.StructureStart.m_226850_(StructureStart.java:89)
at TRANSFORMER/[email protected]/net.minecraft.world.level.chunk.ChunkGenerator.m_223080_(ChunkGenerator.java:429)
at TRANSFORMER/[email protected]/net.minecraft.world.level.chunk.ChunkGenerator$$Lambda$51418/0x0000000804f61638.accept(Unknown Source)
at MC-BOOTSTRAP/[email protected]/com.google.common.collect.ImmutableList.forEach(ImmutableList.java:422)
at TRANSFORMER/[email protected]/net.minecraft.world.level.chunk.ChunkGenerator.m_213609_(ChunkGenerator.java:428)
at TRANSFORMER/[email protected]/net.minecraft.world.level.chunk.ChunkStatus.m_223313_(ChunkStatus.java:145)
at TRANSFORMER/[email protected]/net.minecraft.world.level.chunk.ChunkStatus$$Lambda$3714/0x0000000800c37d90.m_214024_(Unknown Source)
at TRANSFORMER/[email protected]/net.minecraft.world.level.chunk.ChunkStatus.m_223279_(ChunkStatus.java:292)
at TRANSFORMER/[email protected]/net.minecraft.server.level.ChunkMap.lambda$scheduleChunkGeneration$25(ChunkMap.java:621)
at TRANSFORMER/[email protected]/net.minecraft.server.level.ChunkMap$$Lambda$50028/0x0000000804d64b98.apply(Unknown Source)
at MC-BOOTSTRAP/[email protected]/com.mojang.datafixers.util.Either$Left.map(Either.java:38)
at TRANSFORMER/[email protected]/net.minecraft.server.level.ChunkMap.lambda$scheduleChunkGeneration$27(ChunkMap.java:619)
at TRANSFORMER/[email protected]/net.minecraft.server.level.ChunkMap$$Lambda$50027/0x0000000804d64958.apply(Unknown Source)
at [email protected]/java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:1150)
at [email protected]/java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:482)
at TRANSFORMER/[email protected]/net.minecraft.server.level.ChunkTaskPriorityQueueSorter.m_143188_(ChunkTaskPriorityQueueSorter.java:62)
at TRANSFORMER/[email protected]/net.minecraft.server.level.ChunkTaskPriorityQueueSorter$$Lambda$48663/0x0000000804b0f650.run(Unknown Source)
at TRANSFORMER/[email protected]/net.minecraft.util.thread.ProcessorMailbox.m_18759_(ProcessorMailbox.java:91)
at TRANSFORMER/[email protected]/net.minecraft.util.thread.ProcessorMailbox.m_18747_(ProcessorMailbox.java:146)
at TRANSFORMER/[email protected]/net.minecraft.util.thread.ProcessorMailbox.run(ProcessorMailbox.java:102)
at [email protected]/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1395)
at [email protected]/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373)
at [email protected]/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182)
at [email protected]/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655)
at [email protected]/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622)
at [email protected]/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165)