Unable to access pages

Hi,
We are having problems with our yoast plugin on our wordpress site. When it is active we are unable to access the pages in the back end of the site to edit – instead we get this error message:

Warning: preg_match_all(): Empty regular expression in /home/helliwel/public_html/wp-content/plugins/wordpress-seo/inc/class-wpseo-replace-vars.php on line 164

As I understand It this is due to the php on our site. I have already emailed our host and they have comeback with this response.

As I am able to see on the screenshot that you have provided, the PHP Time Limit is actually the Max Execution Time, which is already increased to 600:
http://79.170.40.160/helliwellmedia.com/info.php

Please make sure that you are checking the values in your server with us, not on your live website. The values in here:

http://79.170.40.160/helliwellmedia.com/info.php

are affecting all the files/scripts on our server for that particular domain: helliwellmedia.com
If this is not taking effect for particular plugin, you would need to further check with the developer to exclude the possibility for caching plugin and to check the php constants in wp-includes.

Please can anyone advise what we need to do to get this working
Thanks
Lindsay