The server crashes when a minecart goes through a cart assembler. Pasting the server error below + adding crash logs.
crash-2020-06-13_20.15.06-server.txt

[13Jun2020 20:14:05.914] [Server thread/ERROR] [net.minecraft.server.MinecraftServer/]: Encountered an unexpected exception net.minecraft.crash.ReportedException: Ticking entity at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:853) ~[?:?] at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:330) ~[?:?] at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:784) ~[?:?] at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:637) [?:?] at java.lang.Thread.run(Thread.java:748) [?:1.8.0_252] Caused by: java.lang.NoSuchMethodError: net.minecraft.entity.item.minecart.AbstractMinecartEntity.func_70016_h(DDD)V at com.simibubi.create.content.contraptions.components.structureMovement.mounted.CartAssemblerBlock.lambda$onMinecartPass$0(CartAssemblerBlock.java:94) ~[?:mc1.15.2_v0.2.4] at com.simibubi.create.foundation.block.ITE.withTileEntityDo(ITE.java:22) ~[?:mc1.15.2_v0.2.4] at com.simibubi.create.content.contraptions.components.structureMovement.mounted.CartAssemblerBlock.onMinecartPass(CartAssemblerBlock.java:84) ~[?:mc1.15.2_v0.2.4] at net.minecraft.entity.item.minecart.AbstractMinecartEntity.func_180460_a(AbstractMinecartEntity.java:449) ~[?:?] at net.minecraft.entity.item.minecart.AbstractMinecartEntity.func_70071_h_(AbstractMinecartEntity.java:245) ~[?:?] at net.minecraft.world.server.ServerWorld.func_217479_a(ServerWorld.java:600) ~[?:?] at net.minecraft.world.World.func_217390_a(World.java:536) ~[?:?] at net.minecraft.world.server.ServerWorld.func_72835_b(ServerWorld.java:401) ~[?:?] at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:849) ~[?:?] ... 4 more

Hi, this is known and fixed- Try spinning up Create 0.2.4c on your server.
Thanks for reporting!