Exception with queued tracking - I suspect the queuedTracking Plugin to be the reason for the Exception:
Uncaught exception in /home/users/xxx/www/stats/core/DeviceDetector/DeviceDetectorFactory.php line 28:
Piwik\DeviceDetector\DeviceDetectorFactory::makeInstance(): Argument #2 ($clientHints) must be of type array, null given, called in /home/users/xxx/www/stats/core/Tracker/VisitExcluded.php on line 193
When updated from an older version, requests of an "older format" may have been still present in the queue and could not be processed further.
Piwik/Request getClientHints() should return an empty array, if no data was found, not null, because for DeviceDetectorFactory::makeInstance(), $clientHints has to be an array.
Exception is thrown, Queue handling stops.
Could this be the same as https://github.com/matomo-org/plugin-QueuedTracking/issues/192 and the linked forum thread there?