This Exception shows the error:
java.lang.UnsupportedClassVersionError: org/jivesoftware/openfire/plugin/ImportExportPlugin : Unsupported major.minor version 51.0
These are the Java version numbers and binary version numbers
J2SE 8=52,
J2SE 7=51,
J2SE 6.0=50,
J2SE 5.0=49,
JDK 1.4=48,
JDK 1.3=47,
JDK 1.2=46,
JDK 1.1=45