Try to delete the plugin, which should be in the directory "~/.jabref\plugins".

This shouldn't happen due to the plugin, but in any case you should not install this plugin, as its contents have been superseded by built-in functionality in JabRef 2.9.2.

If deleting the plugin doesn't work, I would try to reset your configuration, by moving or deleting the file ~/.java/.userPrefs/net/sf/jabref/prefs.xml.