Error connecting to DB in /wp-includes/wp-db.php on line 1538 [closed]

Are you running on Ubuntu? Library libc was recently updated, and if you have unattended-upgrades enabled, that will be the cause. A restart of PHP-FPM, Apache, Nginx will solve it. Even better if you could reboot the entire server to be sure.

libc is the C implementation for Linux, and PHP runs on C.

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