After upgrading from 1.8.4 to 1.9.0, Throttle Concurrent Builds Plug-in started displaying following error:

FINEST|20230/0|Service jenkins|16-04-15 10:44:35|java.lang.InstantiationException: java.lang.UnsupportedClassVersionError: hudson/plugins/throttleconcurrents/ThrottleMatrixProjectOptions : unsupported classversion 51.0

We use JDK6 and this plugin does not document that it requires JDK7.

We use latest required Jenkins core 1.609.3, which does not require JDK7 either.

The fix could be as simple as regenerate plugin with a JDK6.