Broken after update 4.4

Hi Guys,

After updating WordPress on my website , it broke down and the following message appears:
Fatal error: Cannot redeclare preview_theme() (previously declared in /public/sites/www.slangenburg-meditatie.nl/wp-includes/theme.php:650) in /public/sites/www.slangenburg-meditatie.nl/wp-includes/deprecated.php on line 3414

I disabled W3 total cache already but can’t find out how to possibly fix this. Is there a way to fix this or can I downgrade to 4.3.1?

Please let me know.

Hindrik