The colour scheme for our theme has previously been pink but has reverted to the default grey colour scheme for that theme. When I go to check the theme customization settings, it still show the pink scheme but when viewing the page it is the default grey colour scheme. When logging in to the site I get this error message along the bottom of the login page....

Warning: file_put_contents(/home/volunteercentrewiltshireorguk/www/files/cache/pages/l/o/g/login.cache): failed to open stream: Permission denied in /home/volunteercentrewiltshireorguk/www/updates/concrete5.6.3.1_updater/concrete/core/libraries/page_cache/types/file.php on line 67

How can I fix this bug? I manage the website for a charity organisation and would dearly like to fix the issue. The last back up was before a few recent additions so avoiding restore would be great if possible.