How to fix:
Object of class WP_Error could not be converted to string in /wp-includes/rewrite.php on line 326
All you need to do is to open phpMyadmin and go to the category_base option within the wp_options table.
Empty, and save it.
About Ovidiu Purdea
Ovidiu Purdea, WordPress Webmaster and SEO & SEM specialist with over 9 years experience in this field. Guru using, modifying, adapting websites based on WordPress. You can follow Ovidiu Purdea on my Google+ profile.