Uncaught PHP Exception Symfony\Component\Cache\Exception\InvalidArgumentException: "Cache key must be string, "NULL" given"

[s.elettronew]

Hi guys,I had a problem that actually is solved, but i want understand what is happen.
The site give me no reponse, white page and i contacted the host and they answed me that there is no problem of server, but is an applicative error.I checked the log and i found this : request.CRITICAL: Uncaught PHP Exception Symfony\Component\Cache\Exception\InvalidArgumentException: "Cache key must be string, "NULL" given" at /var/www/vhosts/elettronew.com/httpdocs/vendor/symfony/symfony/src/Symfony/Component/Cache/CacheItem.php line 156 {"exception":"[object] (Symfony\\Component\\Cache\\Exception\\InvalidArgumentException(code: 0): Cache key must be string, \"NULL\" given at /var/www/vhosts/elettronew.com/httpdocs/vendor/symfony/symfony/src/Symfony/Component/Cache/CacheItem.php:156)"} []Then i have enabled the dev mode and i get this on my white page: SELECT l.*, ls.`id_shop` FROM `ww_ps_lang` l LEFT JOIN `ww_ps_lang_shop` ls ON (l.id_lang = ls.id_lang) ( so i thought that there was a db problem, but a generic sql request work correctly, only this one not worked)After some minutes, the site is running normally. According to you, what is happen?