Just go to phpmyadmin and in the xx_configuration table add that record. It should fix the "error" although you can also "disable" it by changing the value of "PS_ALLOW_MOBILE_DEVICE" in that same table and setting it to 0 (not sure where/if it is configurable from the BO - probably under the theme settings).
From the Context class:
return isset($_SERVER['HTTP_USER_AGENT'], Context::getContext()->cookie) && (bool) Configuration::get('PS_ALLOW_MOBILE_DEVICE') && defined('_PS_THEME_MOBILE_DIR_') && @filemtime(_PS_THEME_MOBILE_DIR_) && !Context::getContext()->cookie->no_mobile;
As I understand it, you would only enable this if you have a specific, separate mobile theme.