On entering the multiplayer window, I can see the server as available.

Attempting to connect to the server fails at "Logging in".
Server shows no logs showing the attempt, client reports " Connecting to [server], [port]"
Clicked cancel after 7 minutes.

Clicked refresh and attempted to reconnect, and it logged in, but in less than 60 seconds I get "internal server error" from the client and the server reports:

[19:31:53] [Server thread/WARN]: Failed to handle packet for /[IP]:51800
java.lang.RuntimeException: Already decorating!!
at agj.a(SourceFile:21) ~[minecraft_server.13w42a.jar:?]
at agd.a(SourceFile:355) ~[minecraft_server.13w42a.jar:?]
at agy.a(SourceFile:74) ~[minecraft_server.13w42a.jar:?]
at ape.a(SourceFile:432) ~[minecraft_server.13w42a.jar:?]
at mh.a(SourceFile:160) ~[minecraft_server.13w42a.jar:?]
at aoc.a(SourceFile:778) ~[minecraft_server.13w42a.jar:?]
at mh.c(SourceFile:95) ~[minecraft_server.13w42a.jar:?]
at mg.<init>(SourceFile:31) ~[minecraft_server.13w42a.jar:?]
at mf.a(SourceFile:212) ~[minecraft_server.13w42a.jar:?]
at mf.d(SourceFile:346) ~[minecraft_server.13w42a.jar:?]
at lc.d(SourceFile:215) ~[minecraft_server.13w42a.jar:?]
at mv.a(SourceFile:273) ~[minecraft_server.13w42a.jar:?]
at iq.a(SourceFile:137) ~[minecraft_server.13w42a.jar:?]
at ir.a(SourceFile:63) ~[minecraft_server.13w42a.jar:?]
at el.a(SourceFile:145) ~[minecraft_server.13w42a.jar:?]
at mr.c(SourceFile:134) [minecraft_server.13w42a.jar:?]
at net.minecraft.server.MinecraftServer.u(SourceFile:529) [minecraft_server.13w42a.jar:?]
at li.u(SourceFile:259) [minecraft_server.13w42a.jar:?]
at net.minecraft.server.MinecraftServer.t(SourceFile:441) [minecraft_server.13w42a.jar:?]
at net.minecraft.server.MinecraftServer.run(SourceFile:358) [minecraft_server.13w42a.jar:?]
at kx.run(SourceFile:617) [minecraft_server.13w42a.jar:?]
[19:31:53] [Server thread/INFO]: [USER] lost connection: TextComponent{text='Internal server error', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null}}
[19:31:53] [Server thread/INFO]: [USER] left the game

Attempting to reconnect at this point, the client reports:

Client> 18/10/2013 7:33:49 PM io.netty.channel.nio.NioEventLoop select
Client> WARNING: Selector.select() returned prematurely 512 times in a row; rebuilding selector.
Client> 18/10/2013 7:33:49 PM io.netty.channel.nio.NioEventLoop rebuildSelector
Client> INFO: Migrated 1 channel(s) to the new Selector.
Client> [19:33:49] Server Connector #3/ERROR: Couldn't connect to server
Client> java.nio.channels.CancelledKeyException
Client> at sun.nio.ch.SelectionKeyImpl.ensureValid(SelectionKeyImpl.java:55) ~[?:1.6.0_26]
Client> at sun.nio.ch.SelectionKeyImpl.interestOps(SelectionKeyImpl.java:64) ~[?:1.6.0_26]
Client> at io.netty.channel.socket.nio.NioSocketChannel.doConnect(NioSocketChannel.java:178) ~[netty-all-4.0.10.Final.jar:?]
Client> at io.netty.channel.nio.AbstractNioChannel$AbstractNioUnsafe.connect(AbstractNioChannel.java:169) ~[netty-all-4.0.10.Final.jar:?]
Client> at io.netty.channel.DefaultChannelPipeline$HeadHandler.connect(DefaultChannelPipeline.java:1008) ~[netty-all-4.0.10.Final.jar:?]
Client> at io.netty.channel.DefaultChannelHandlerContext.invokeConnect(DefaultChannelHandlerContext.java:494) ~[netty-all-4.0.10.Final.jar:?]
Client> at io.netty.channel.DefaultChannelHandlerContext.connect(DefaultChannelHandlerContext.java:479) ~[netty-all-4.0.10.Final.jar:?]
Client> at io.netty.channel.ChannelOutboundHandlerAdapter.connect(ChannelOutboundHandlerAdapter.java:47) ~[netty-all-4.0.10.Final.jar:?]
Client> at io.netty.channel.DefaultChannelHandlerContext.invokeConnect(DefaultChannelHandlerContext.java:494) ~[netty-all-4.0.10.Final.jar:?]
Client> at io.netty.channel.DefaultChannelHandlerContext.connect(DefaultChannelHandlerContext.java:479) ~[netty-all-4.0.10.Final.jar:?]
Client> at io.netty.channel.ChannelOutboundHandlerAdapter.connect(ChannelOutboundHandlerAdapter.java:47) ~[netty-all-4.0.10.Final.jar:?]
Client> at io.netty.channel.DefaultChannelHandlerContext.invokeConnect(DefaultChannelHandlerContext.java:494) ~[netty-all-4.0.10.Final.jar:?]
Client> at io.netty.channel.DefaultChannelHandlerContext.connect(DefaultChannelHandlerContext.java:479) ~[netty-all-4.0.10.Final.jar:?]
Client> at io.netty.channel.DefaultChannelHandlerContext.connect(DefaultChannelHandlerContext.java:464) ~[netty-all-4.0.10.Final.jar:?]
Client> at io.netty.channel.DefaultChannelPipeline.connect(DefaultChannelPipeline.java:847) ~[netty-all-4.0.10.Final.jar:?]
Client> at io.netty.channel.AbstractChannel.connect(AbstractChannel.java:198) ~[netty-all-4.0.10.Final.jar:?]
Client> at io.netty.bootstrap.Bootstrap$2.run(Bootstrap.java:165) ~[netty-all-4.0.10.Final.jar:?]
Client> at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:354) ~[netty-all-4.0.10.Final.jar:?]
Client> at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:348) ~[netty-all-4.0.10.Final.jar:?]
Client> at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:101) ~[netty-all-4.0.10.Final.jar:?]
Client> at java.lang.Thread.run(Thread.java:662) ~[?:1.6.0_26]