nuxeo-5.1.0.M2: NoClassDefFoundError -SystemSession

Hi, I just downloaded nuxeo-5.1.0.M2: on startup, I get a 

java.lang.NoClassDefFoundError: or/nuxeo/ecm/platform/api/login/SystemSession 

from the module nuxeo-platform-search-facade-5.1.0.M2.jar. 

Therefore, the webapp does not start and I get an Exception when I try to access it. 

Is there a jar missing from the deployment or does anyone have an idea, what has gone wrong here?! 

Thx for the help
stf

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