Moodle session_start returns FALSE

I need some help installing Moodle v3.7.2. I've passed all checks for server configuration during installation, except for the fact that the site is recognized as http instead of https, maybe because of the proxy that sits in front of the nginx serving Moodle. Anyway, when I try to load the first page, I receive a generic error: $string['servererror'] = 'An error occurred whilst communicating with the server'; I've investigated the source code for this error, and found the motivation: On path /moodle/lib/classes/session/manager.php, row 90, this check fails: if (!self::$handler->start()) The start() method is simply calling a php function: session_start(); That returns FALSE and throw the exception. Any idea on how to solve this? Many thanks. EDIT: I've also tried storing sessions on database: $CFG->session_handler_class = '\core\session\database'; $CFG->session_database_acquire_lock_timeout = 120; But with no luck, now the function that fails is: session_set_save_handler