Hello,
i have the same problem.
I installed YOAST and activate it and it works well… then i deactivate it and 10 minutes later i want to reactivate it again and i got the same error.
Fatal error: Class ‘WPSEO_OnPage’ not found in /DOMAIN/subdomains/www/html/wp-content/plugins/wordpress-seo/wp-seo-main.php on line 353
