To me, the problem is cause by a change (by the hosting company) in the absolute path of the server. By following the instruction in the warning, I solve the problem. No need to touch the wp-config.php file.
Open the file : /wp-content/advanced-cache.php
Compare the absolute path written in that file with the one include in the warning message (in Wordpress dashboard). The warning message is giving you the new path, use it to fix the one in the advanced-cache.php file.