Deprecated: Non-static method BackWPup_Admin::admin_css() should not be called statically in /var/www/html/wp-includes/class-wp-hook.php on line 292

I'm not sure what this is. There's a "banner" across the top of my localhost dev site. It just appeared in the last two weeks.

Here's the message again:

Deprecated: Non-static method BackWPup_Admin::admin_css() should not be called statically in /var/www/html/wp-includes/class-wp-hook.php on line 292

Except for naming the backup and selecting the local folder to back up to and making sure the permission are writable for that directory, all the settings remain at default.

Here's the whole programming thing from class-wp-hook.php from line 269 to line 305:

`public function apply_filters( $value, $args ) {
if ( ! $this->callbacks ) {
return $value;
}

	$nesting_level = $this->nesting_level++;

	$this->iterations[ $nesting_level ] = array_keys( $this->callbacks );
	$num_args                           = count( $args );

	do {
		$this->current_priority[ $nesting_level ] = current( $this->iterations[ $nesting_level ] );
		$priority                                 = $this->current_priority[ $nesting_level ];

		foreach ( $this->callbacks[ $priority ] as $the_ ) {
			if ( ! $this->doing_action ) {
				$args[0] = $value;
			}

			// Avoid the array_slice() if possible.
			if ( 0 == $the_['accepted_args'] ) {
				$value = call_user_func( $the_['function'] );
			} elseif ( $the_['accepted_args'] >= $num_args ) {
				$value = call_user_func_array( $the_['function'], $args );
			} else {
				$value = call_user_func_array( $the_['function'], array_slice( $args, 0, (int) $the_['accepted_args'] ) );
			}
		}
	} while ( false !== next( $this->iterations[ $nesting_level ] ) );

	unset( $this->iterations[ $nesting_level ] );
	unset( $this->current_priority[ $nesting_level ] );

	$this->nesting_level--;

	return $value;
}`

Start with the reported call in class-wp-hook.php and trace where BackWPup_Admin::admin_css() is registered as a callback. Check the plugin's admin CSS path and reproduce the notice in the reported WordPress/PHP environment; done means the deprecation message no longer appears.