Function Error after WP Upgrade to 3.9.1

You should contact the plugin author, because plugin support is generally off topic on this site.

But … I got curious, peeked into the plugin code and found this line:

add_action( 'wp_head', array('dc_jqverticalmegamenu', 'header') );

where the header() method is assumed static, but it’s not:

function header(){
    // ...
}

That’s why this strict notice pops up.

The plugin creates an instance of the class through:

  // Initialize the plugin.
 $dcjqverticalmegamenu = new dc_jqverticalmegamenu();

so you should instead try:

add_action( 'wp_head', array( $this, 'header') );

and similar for the other static calls.