I too am getting the same error with the free version after updating to the latest version (and updating to same day update).
The issue stops only after disabling the Yoast SEO plugin.
Warning: strpos() [function.strpos]: Empty needle in /home/magetemp/public_html/blog/wp-content/plugins/wordpress-seo/admin/class-database-proxy.php on line 44
Warning: strpos() [function.strpos]: Empty needle in /home/magetemp/public_html/blog/wp-content/plugins/wordpress-seo/admin/class-database-proxy.php on line 44
Warning: strpos() [function.strpos]: Empty needle in /home/magetemp/public_html/blog/wp-content/plugins/wordpress-seo/admin/class-database-proxy.php on line 44
Warning: Cannot modify header information - headers already sent by (output started at /home/magetemp/public_html/blog/wp-content/plugins/wordpress-seo/admin/class-database-proxy.php:44) in /home/magetemp/public_html/blog/wp-admin/includes/misc.php on line 1116