Tekkit Server, Clients Stuck On Logging In

[Zechlyr]

Hello,

I made a Tekkit server for my friends and I to play on.  I can connect to it no problem, but my friends are stuck on the logging in screen.  When my friend tries to join, an error message displays on the server log.  It says:

 

2016-06-18 20:02:18 [INFO] [STDERR] java.io.IOException: Bad packet id 17
2016-06-18 20:02:18 [INFO] [STDERR]     at net.minecraft.network.packet.Packet.func_73272_a(Packet.java:149)
2016-06-18 20:02:18 [INFO] [STDERR]     at net.minecraft.network.TcpConnection.func_74447_i(TcpConnection.java:325)
2016-06-18 20:02:18 [INFO] [STDERR]     at net.minecraft.network.TcpConnection.func_74450_c(TcpConnection.java:549)
2016-06-18 20:02:18 [INFO] [STDERR]     at net.minecraft.network.TcpReaderThread.run(SourceFile:94)
2016-06-18 20:02:18 [INFO] [Minecraft-Server] /(friend's IP:port) lost connection

 

However, when I ran my vanilla server, my friends could join with no problems at all.  Offline mode is set to false.

Help?