Minecraft 13w16a is stuck on Mojang Screen

Whenever I try to play the new snapshot (13w16a) from the new launcher, the game gets stuck at the mojang screen. The last few lines in the launcher's dialogue are:
Client> java.lang.StringIndexOutOfBoundsException: String index out of range: -1
Client> at java.lang.String.substring(String.java:1937)
Client> at asm.a(SourceFile:1469)
Client> at asm.a(SourceFile:1461)
Client> at asm.k(SourceFile:1452)
Client> at asm.K(SourceFile:300)
Client> at asm.c(SourceFile:503)
Client> at net.minecraft.client.main.Main.main(SourceFile:76)
Client> b@3a2cd728

the last line (b@3a2cd728) are different each time I try to play. I've deleted my ~/Application\ Support/minecraft directory and started clean from the new launcher.