title: Why after downgrading from Nexus Professional to Nexus OSS it says "Unable to lookup security reals"?
body: <p>After downgrading from Nexus Professional to OSS, Nexus starts but with an exception in [nexus-runtime-oss]/logs/wrapper.log</p>
  
  <pre><code>INFO  [jetty-main-1   ] - org.sonatype.security.configuration.source.FileSecurityConfigurationSource - Loading Security configuration from /var/opt/jet/nexus-data/sonatype-work/nexus/conf/security-configuration.xml
  ERROR [jetty-main-1   ] - org.sonatype.security.DefaultSecuritySystem - Unable to lookup security realms
  java.lang.ClassNotFoundException: User-Token-Realm
  |      at java.net.URLClassLoader.findClass(URLClassLoader.java:381) ~[na:1.8.0_66]
  |      at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[na:1.8.0_66]
  </code></pre>