I have the same exact problem, and it's definitely not a hamatchi error, as I don't use hamatchi. The error is the same,
"2013-12-18 16:01:50 [SEVERE] [arsmagica2] The language resource /lang/de_DE.lang cannot be located on the classpath. This is a programming error."
It's happened since 1.0.2 and is still happening in 1.0.4[DOUBLEPOST=1387412022][/DOUBLEPOST]And I too cannot load singleplayer now[DOUBLEPOST=1387412142][/DOUBLEPOST]One more thing, the "world" file and all the text files (server.properties, ops, whitelist, etc.) do not appear; I have to add them manually[DOUBLEPOST=1387412250][/DOUBLEPOST]
http://pastebin.com/upVvEajt Here is the crash log