Here's the exception (this is also the entire contents of spine.log):

$ spine
Spine Launcher 1.2.1-full
Licensed to: x
Linux amd64 3.10.7-1-ARCH
Java 1.7.0-u6-unofficial Oracle Corporation
OpenJDK 64-Bit Server VM
Up to date: 1.5.12
Spine 1.5.12-full, LWJGL
Error starting app.
java.lang.reflect.InvocationTargetException
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:601)
	at l.g.run(SourceFile:677)
	at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:241)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:711)
	at java.awt.EventQueue.access$000(EventQueue.java:104)
	at java.awt.EventQueue$3.run(EventQueue.java:672)
	at java.awt.EventQueue$3.run(EventQueue.java:670)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:681)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:244)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:163)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:147)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:139)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:97)
Caused by: java.lang.IllegalArgumentException: Field "skinPlaceholder" not found on class: xp
	at com.esotericsoftware.kryo.serializers.FieldSerializer.getField(SourceFile:235)
	at zs.<init>(SourceFile:137)
	at com.esotericsoftware.spine.editor.Editor.<init>(SourceFile:119)
	at com.esotericsoftware.spine.editor.Editor.main(SourceFile:1398)
	... 19 more