I was doing some testing this morning with the WP Supercache plugin and I checked the box "Coarse file locking" in the Advance Settings page. This caused the Yoast SEO plugin to display this error message :
<meta name="description" content="<?php bloginfo('description'); ?>" /> `.
So I unchecked that box, went back to the Yoast SEO plugin and I clicked on the red "Fix" button. The error disappeared and I saved the setting.
If you have already tried to fix the problem by clicking on the red "Fix" button then it might not work but try renaming or deleting the .bak file that was created when your first clicked on the red "fix" button - something like: header.php.wpseobak - in your theme directory, and then go back to the yoast SEO plugin and try to fix it again using the red "Fix" button.
Hope this will help someone.