Client cannot connect to internal or external server

This may be a dupe, but I looked for a while and had trouble finding anything. Essentially, loading singleplayer worlds in the latest snapshot fails for me every time with the following error:

java.lang.IllegalStateException: failed to create a child event loop
	at io.netty.util.concurrent.MultithreadEventExecutorGroup.<init>(MultithreadEventExecutorGroup.java:68)
	at io.netty.channel.MultithreadEventLoopGroup.<init>(MultithreadEventLoopGroup.java:49)
	at io.netty.channel.nio.NioEventLoopGroup.<init>(NioEventLoopGroup.java:61)
	at io.netty.channel.nio.NioEventLoopGroup.<init>(NioEventLoopGroup.java:52)
	at ln$1.a(SourceFile:44)
	at ln$1.b(SourceFile:41)
	at nq.c(SourceFile:10)
	at ln.a(SourceFile:121)
	at axd.a(SourceFile:1925)
	at azw.i(SourceFile:172)
	at azw$a.a(SourceFile:224)
	at ayj.p(SourceFile:315)
	at azw.k(SourceFile:82)
	at azv.p(SourceFile:380)
	at axd.s(SourceFile:1476)
	at axd.at(SourceFile:874)
	at axd.a(SourceFile:323)
	at net.minecraft.client.main.Main.main(SourceFile:125)
Caused by: io.netty.channel.ChannelException: failed to open a new selector
	at io.netty.channel.nio.NioEventLoop.openSelector(NioEventLoop.java:128)
	at io.netty.channel.nio.NioEventLoop.<init>(NioEventLoop.java:120)
	at io.netty.channel.nio.NioEventLoopGroup.newChild(NioEventLoopGroup.java:87)
	at io.netty.util.concurrent.MultithreadEventExecutorGroup.<init>(MultithreadEventExecutorGroup.java:64)
	... 17 more
Caused by: java.io.IOException: Unable to establish loopback connection
	at sun.nio.ch.PipeImpl$Initializer.run(PipeImpl.java:101)
	at sun.nio.ch.PipeImpl$Initializer.run(PipeImpl.java:68)
	at java.security.AccessController.doPrivileged(Native Method)
	at sun.nio.ch.PipeImpl.<init>(PipeImpl.java:170)
	at sun.nio.ch.SelectorProviderImpl.openPipe(SelectorProviderImpl.java:50)
	at java.nio.channels.Pipe.open(Pipe.java:155)
	at sun.nio.ch.WindowsSelectorImpl.<init>(WindowsSelectorImpl.java:127)
	at sun.nio.ch.WindowsSelectorProvider.openSelector(WindowsSelectorProvider.java:44)
	at io.netty.channel.nio.NioEventLoop.openSelector(NioEventLoop.java:126)
	... 20 more
Caused by: java.net.ConnectException: Connection refused: connect
	at sun.nio.ch.Net.connect0(Native Method)
	at sun.nio.ch.Net.connect(Net.java:457)
	at sun.nio.ch.Net.connect(Net.java:449)
	at sun.nio.ch.SocketChannelImpl.connect(SocketChannelImpl.java:647)
	at java.nio.channels.SocketChannel.open(SocketChannel.java:189)
	at sun.nio.ch.PipeImpl$Initializer$LoopbackConnector.run(PipeImpl.java:130)
	at sun.nio.ch.PipeImpl$Initializer.run(PipeImpl.java:83)
	... 28 more

I'm pretty sure this means that the client failed to create a child thread for netty. Anyways, sorry if this is a duplicate, but I thought I'd report it as I didn't see any other reports related to this. Also, this error occurs when attempting to join an external server:

java.lang.NullPointerException: group
	at io.netty.bootstrap.AbstractBootstrap.group(AbstractBootstrap.java:80) ~[netty-all-4.0.23.Final.jar:4.0.23.Final]
	at ek.a(SourceFile:264) ~[15w31c.jar:?]
	at aza$1.run(SourceFile:59) [15w31c.jar:?]