Alain Raap

@Nadav thanks for your message, I search with Google and also in the forums here when I have a problem with our Moodle stack, but some questions are difficult to post, also because not all information I can share here about our infrastructure. The wiki page I also visited in the past, but the information was to outdated then (Moodle versions 2.x postings) and are not representative for this time anymore.

I can't visit the chatlogs link, isn't it available yet? Thanks for sharing the content anyway!

Thanks Howard for your explanation. I checked the code that throws the error and thought it might be a problem,
this is the code snippet of lib/classes/session/manager.php I found:

*
* @param bool $newsid is this a new session in first http request?
*/
protected static function initialise_user_session($newsid) {
global $CFG, $DB;

$sid = session_id();
if (!$sid) {
// No session, very weird.
error_log('Missing session ID, session not started!');
self::init_empty_session();
return;
}

if (!$record = $DB->get_record('sessions', array('sid'=>$sid), 'id, sid, state, userid, lastip, timecreated, timemodified')) {
if (!$newsid) {
if (!empty($_SESSION['USER']->id)) {
// This should not happen, just log it, we MUST not produce any output here!
error_log("Cannot find session record $sid for user ".$_SESSION['USER']->id.", creating new session.");
}
// Prevent session fixation attacks.
session_regenerate_id(true);
}
$_SESSION = array();
}
unset($sid);

Good to know PHP 7.4 works fine now, and yes, purching caches always is a good solution for many problems (after upgrading).

We also recently upgraded from Moodle 3.5 to 3.9, with PHP 7.2 Moodle runs without problems. Which database and which version do you use?
Did you also check all the messages which you can see in the Admin console if all your requirements are OK?