purchased composition plugin not enabled

Question

Bought composition plugin, upgraded to version that requires payment, entered license key, clicked "enable", see this message:

This add-on couldn't be loaded. It has one or more errors that prevent it from being enabled.

No errors listed, no links to logs, nothing in catalina.out.

Running Confluence 5.6.1.

Chris Walquist · Answer

Never mind...Google told me that "Unsupported major.minor version 51.0" translates to "You're running on a too-old Java".

Chris Walquist · Answer

Found this error after turning on "Diagnostic" log level:

2015-07-30 10:06:50,408 ERROR [http-bio-8090-exec-18653] [atlassian.plugin.manager.DefaultPluginManager] enableConfiguredPluginModule There was an error loading the descriptor 'deck' of plugin 'net.customware.confluence.plugin.composition'. Disabling.
 -- url: /rest/plugins/1.0/net.customware.confluence.plugin.composition-key | userName: cwalquist | referer: https://wiki.drwholdings.com/plugins/servlet/upm
java.lang.UnsupportedClassVersionError: net/customware/confluence/plugin/composition/macro/DeckMacro : Unsupported major.minor version 51.0