If you have troubles since the update to WordPress 4.5 please read this post before opening a new thread.
Please go to Appearance > Editor and open 404.php file and search for following lines
$avia_config['currently_viewing'] = 'page'; get_sidebar();
then comment them out as following
//$avia_config['currently_viewing'] = 'page'; //get_sidebar();
Then search for following line
<?php avia_layout_class( 'content' ); ?>
and change it to
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