Error showing for Category

My website is www.themamaneedscake.com and I am trying to add categories to my menu and I am getting this error message:

Fatal error : ob_start(): Cannot use output buffering in output buffering display handlers in /home/themaman/public_html/wp-content/plugins/wordpress-seo/frontend/class-frontend.php on line 1781

Can this be fixed?

Fixed my own issue - there were 2 SEO plug-ins activated and it was causing the problem.