So me and my friend have been playing FTB Tech World 2. And for some reason when both of us are on the server crashes. When he is on the server by himself the server doesn't crash. I copied the log in the console hopefully this helps your guys. It's in the spoiler.
12018-06-20 01:34:44 [INFO] Starting minecraft server version 1.6.4
22018-06-20 01:35:32 [INFO] Loading properties
32018-06-20 01:35:32 [INFO] Default game type: SURVIVAL
42018-06-20 01:35:32 [INFO] Generating keypair
52018-06-20 01:35:32 [INFO] Starting Minecraft server on *:40674
62018-06-20 01:35:46 [INFO] Preparing level "world"
72018-06-20 01:35:48 [INFO] Preparing start region for level 0
82018-06-20 01:35:49 [INFO] Preparing spawn area: 36%
92018-06-20 01:35:50 [INFO] Done (3.670s)! For help, type "help" or "?"
102018-06-20 01:35:50 [INFO] Starting GS4 status listener
112018-06-20 01:35:50 [INFO] Query running on **.**.**.**:40674
122018-06-20 01:36:00 [SEVERE] Reached end of stream for /**.**.**.**
132018-06-20 01:36:13 [SEVERE] Reached end of stream for /**.**.**.**
142018-06-20 01:36:17 [INFO] WarmerGem12[/**.**.**.**:51715] logged in with entity id 397 at (-249.89702783105963, 72.0, 668.0222624296398)
152018-06-20 01:36:17 [INFO] WarmerGem12 joined the game
162018-06-20 01:36:23 [WARNING] Can't keep up! Did the system time change, or is the server overloaded?
172018-06-20 01:36:46 [WARNING] Can't keep up! Did the system time change, or is the server overloaded?
182018-06-20 01:37:05 [INFO] WarmerGem12 was slain by Zombie
192018-06-20 01:39:20 [WARNING] Can't keep up! Did the system time change, or is the server overloaded?
202018-06-20 01:40:45 [INFO] WarmerGem12 was slain by Spider
212018-06-20 01:40:53 [INFO] Saving...
222018-06-20 01:40:54 [INFO] Saved the world
232018-06-20 01:42:40 [SEVERE] Reached end of stream for /**.**.**.**
242018-06-20 01:42:49 [INFO] Chikenmilk[/**.**.**.**:51171] logged in with entity id 9773 at (-169.25914109469227, 66.0, 437.86119618421975)
252018-06-20 01:42:49 [INFO] Chikenmilk joined the game
262018-06-20 01:43:22 [INFO] WarmerGem12 was slain by Spider
272018-06-20 01:43:39 [INFO] Chikenmilk lost connection: disconnect.genericReason
282018-06-20 01:43:39 [INFO] Chikenmilk left the game
292018-06-20 01:45:14 [INFO] WarmerGem12 was slain by Spider
302018-06-20 01:45:53 [INFO] Successfully set the idle timeout to 10 minutes.
312018-06-20 01:45:53 [INFO] Saving...
322018-06-20 01:45:54 [INFO] Saved the world
332018-06-20 01:49:50 [WARNING] Can't keep up! Did the system time change, or is the server overloaded?
342018-06-20 01:50:54 [INFO] Saving...
352018-06-20 01:50:54 [INFO] Saved the world
362018-06-20 01:52:49 [SEVERE] Reached end of stream for /**.**.**.**
372018-06-20 01:52:55 [INFO] Chikenmilk[/**.**.**.**:51328] logged in with entity id 34257 at (-169.25914109469227, 66.0, 437.86119618421975)
382018-06-20 01:52:55 [INFO] Chikenmilk joined the game
392018-06-20 01:52:57 [WARNING] Can't keep up! Did the system time change, or is the server overloaded?
402018-06-20 01:53:36 [WARNING] Can't keep up! Did the system time change, or is the server overloaded?
412018-06-20 01:53:40 [SEVERE] Encountered an unexpected exception NoClassDefFoundError
42java.lang.NoClassDefFoundError: net/minecraft/inventory/InventoryLargeChest
43 at net.minecraft.block.BlockChest.func_94442_h_(BlockChest.java:479)
44 at net.minecraft.block.BlockChest.func_71903_a(BlockChest.java:428)
45 at net.minecraft.item.ItemInWorldManager.func_73078_a(ItemInWorldManager.java:421)
46 at net.minecraft.network.NetServerHandler.func_72472_a(NetServerHandler.java:556)
47 at net.minecraft.network.packet.Packet15Place.func_73279_a(SourceFile:58)
48 at net.minecraft.network.TcpConnection.func_74428_b(TcpConnection.java:462)
49 at net.minecraft.network.NetServerHandler.func_72570_d(NetServerHandler.java:141)
50 at net.minecraft.network.NetworkListenThread.func_71747_b(NetworkListenThread.java:54)
51 at net.minecraft.server.dedicated.DedicatedServerListenThread.func_71747_b(SourceFile:30)
52 at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:691)
53 at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:276)
54 at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:587)
55 at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:484)
56 at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:583)
57Caused by: java.lang.ClassNotFoundException: net.minecraft.inventory.InventoryLargeChest
58 at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:186)
59 at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
60 at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
61 ... 14 more
62Caused by: java.lang.OutOfMemoryError: PermGen space
63 at java.lang.ClassLoader.defineClass1(Native Method)
64 at java.lang.ClassLoader.defineClass(ClassLoader.java:800)
65 at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
66 at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:178)
67 ... 16 more
682018-06-20 01:53:42 [INFO] Stopping server
692018-06-20 01:53:42 [INFO] Saving players
702018-06-20 01:53:46 [INFO] Closing listening thread
712018-06-20 01:53:46 [INFO] WarmerGem12 left the game
722018-06-20 01:54:09 [INFO] Chikenmilk left the game
732018-06-20 01:54:13 [INFO] Saving worlds
742018-06-20 01:54:13 [INFO] Saving chunks for level 'world'/Overworld
752018-06-20 01:54:13 [SEVERE] Reached end of stream for /**.**.**.**
762018-06-20 01:54:14 [INFO] Saving chunks for level 'world'/Nether
