NoClassDefFoundError on startup - does nuxeo run on 1.6?!

I just downloaded nuxeo5.1.0.M2 and I get a NoClassDefFoundError on startup: 

06:25:21,718 WARN  [ServiceController] Problem starting service jboss.j2ee:service=EJB3,module=nuxeo-platform-search-fac
ade-5.1-M2.jar
java.lang.NoClassDefFoundError: org/nuxeo/ecm/platform/api/login/SystemSession
        at java.lang.Class.getDeclaredMethods0(Native Method)
        at java.lang.Class.privateGetDeclaredMethods(Class.java:2427)
        at java.lang.Class.getDeclaredMethods(Class.java:1791)
        at org.jboss.ejb3.injection.PersistenceContextHandler.loadMethodDependencies(PersistenceContextHandler.java:284)

The machine is a windowsXP running jdk 
1.6.0_01 - does nuxeo run on 1.6?! Does anyone have an idea what is wrong here? I'd really like to give nuxeo a try, but it it doesn't even start up...

cheers
stf
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