I just tried to use the DBNavigator plugin.
I restarted IntelliJ and it just hung there which led me on a trip to find out what was wrong.
First up I found the log files in
And saw this exception
Expected implementation for extension declaration (ep = com.intellij.completion.contributor)
This seems to be a general problem for plugins, e.g. the Scala plugin (http://youtrack.jetbrains.net/issue/SCL-1592).
So you need to remove the plugin which will have been installed here:
Thanks to this post for triggering the solution.