If memcached is enabled, it will occasionally write this error to the error_log file.

[29-Jan-2021 07:22:36 Europe/Budapest] PHP Warning: mysqli_connect(): (HY000/1130): Host 'localhost' is not allowed to connect to this MariaDB server in /home/*****/public_html/includes/classes/dbi.class.php on line 87
Where can the error be or what?

The hosting provider told me to try rewriting localhost to 127.0.0.1. I did this in the config file, but the same error.