I was testing for another bug, so I loaded up a world with the default settings in creative (not even changed the name), and this happened.
Seems like the game started generating a world usually

Unfortunately, I can't/don't know how to reproduce the probles as it occurred when creating a standard world with no settings changed.

Log:
[15:41:32] [Server thread/INFO]: Starting integrated minecraft server version 14w30c
[15:41:32] [Server thread/INFO]: Generating keypair
[15:41:32] [Server thread/INFO]: Converting map!
[15:41:32] [Server thread/INFO]: Scanning folders...
[15:41:32] [Server thread/INFO]: Total conversion count is 0
[15:41:32] [Server thread/INFO]: Preparing start region for level 0
[15:41:33] [Server thread/INFO]: Preparing spawn area: 6%
[15:41:34] [Server thread/INFO]: Preparing spawn area: 17%
[15:41:35] [Server thread/INFO]: Preparing spawn area: 28%
[15:41:36] [Server thread/INFO]: Preparing spawn area: 40%
[15:41:37] [Server thread/INFO]: Preparing spawn area: 62%
[15:41:38] [Server thread/INFO]: Preparing spawn area: 78%
[15:41:39] [Server thread/INFO]: Preparing spawn area: 96%
[15:41:40] [Server thread/INFO]: Changing view distance to 16, from 10
[15:41:40] [Server thread/INFO]: DecentM[local:E:846488c8] logged in with entity id 32506375 at (-16.5, 244.0, 36.5)
[15:41:40] [Server thread/INFO]: DecentM joined the game
[15:41:47] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 6774ms behind, skipping 135 tick(s)
[15:42:19] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 17977ms behind, skipping 359 tick(s)
[15:42:29] [Server thread/INFO]: [DecentM: Teleported DecentM to 16.18879365014464, 188.77254852598426, 25.547138371237388]
[15:42:29] [Client thread/INFO]: [CHAT] Teleported DecentM to 16.18879365014464, 188.77254852598426, 25.547138371237388
[15:42:30] [Server thread/INFO]: [DecentM: Teleported DecentM to 16.18879365014464, 238.77254852598426, 25.547138371237388]
[15:42:30] [Client thread/INFO]: [CHAT] Teleported DecentM to 16.18879365014464, 238.77254852598426, 25.547138371237388
[15:42:36] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 3226ms behind, skipping 64 tick(s)
[15:42:51] [Server thread/INFO]: Saving and pausing game...
[15:42:51] [Server thread/INFO]: Saving chunks for level 'New World'/Overworld
java.io.IOException: Stream Closed
at java.io.RandomAccessFile.length(Native Method)
at beq.a(SourceFile:295)
at ber.close(SourceFile:236)
at java.util.zip.DeflaterOutputStream.close(Unknown Source)
at java.io.FilterOutputStream.close(Unknown Source)
at bet.a(SourceFile:162)
at bet.c(SourceFile:150)
at bqk.c(SourceFile:37)
at bqk.run(SourceFile:30)
at java.lang.Thread.run(Unknown Source)
[15:42:52] [Server thread/INFO]: Saving chunks for level 'New World'/Nether
[15:42:52] [Server thread/INFO]: Saving chunks for level 'New World'/The End
[15:42:52] [Server thread/INFO]: DecentM lost connection: TextComponent{text='Disconnected', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null}}
[15:42:52] [Server thread/INFO]: DecentM left the game
[15:42:52] [Server thread/INFO]: Stopping singleplayer server as player logged out
[15:42:52] [Server thread/INFO]: Stopping server
[15:42:52] [Server thread/INFO]: Saving players
[15:42:52] [Server thread/INFO]: Saving worlds
[15:42:52] [Server thread/INFO]: Saving chunks for level 'New World'/Overworld
[15:42:52] [Server thread/INFO]: Saving chunks for level 'New World'/Nether
[15:42:52] [Server thread/INFO]: Saving chunks for level 'New World'/The End