UnsupportedClassVersionError with GWT 2.6 and jdk 1.6

I get this error if I try to debug in super dev mode:

java.lang.UnsupportedClassVersionError: com/google/appengine/tools/development/agent/AppEngineDevAgent : Unsupported major.minor version 51.0
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClassCond(ClassLoader.java:632)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:616)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
    at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
    at sun.instrument.InstrumentationImpl.loadClassAndStartAgent(InstrumentationImpl.java:280)
    at sun.instrument.InstrumentationImpl.loadClassAndCallPremain(InstrumentationImpl.java:338)
FATAL ERROR in native method: processing of -javaagent failed
Exception in thread "main" 

my Eclipse(Mars) runs currently with jdk 1.6_23 because that's currently my Java version thats running on my tomcat. The Compiler is set to 1.6 and the project facets too.

Compile the project with GWT Compile runs fine...

I am working for hours on that problem and I dont see the problem...

Could you help me?

AppEngine requires Java 7. If you're not doing AppEngine-related work, disable it in Eclipse; otherwise update to Java 7.