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Please update the theme to latest version and the afferent plugins, you can check the plugin update status from wp-admin -> appearance -> install plugins.

If this is useless please follow these steps to enable wp debug:

•  Connect to your server by FTP
• Look in root directory of your WordPress install and open wp-config.php
• Search for define( ‘WP_DEBUG’, false ); and change FALSE value to TRUE

define( 'WP_DEBUG', true );

•   After this line add this:

define( 'WP_DEBUG_LOG', true );

WP_DEBUG_LOG is a companion to WP_DEBUG that causes all errors to also be saved to a debug.log log file inside the /wp-content/ directory. This is useful if you want to review all notices later or need to view notices generated off-screen (e.g. during an AJAX request or wp-cron run).

Repeat the described actions and then check the debug.log file and let me know what error appears.

If this is useless please try to deactivate additional plugins except Visual Composer, K-elements, BuddyPress, Paid memberships pro and revolution slider.

After you have disabled the plugins try to see if the error persists.

Cheers