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This might be caused by and outdated version of Java, SonarQube now uses Java 8 as default. You might upgrade java or use an older version of SonarQube.
If you're using intellij plugin of sonarqube, you might have to set the JAVA_HOME ambient variable manually.
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java.lang.UnsupportedClassVersionError: appeng/coremod/AppEngCore : Unsupported major.minor version 52.0
at java.lang.ClassLoader.defineClass1(Native Method)