Can confirm, HTTP500 Server Errors occurred after upgrading TEC to 4.3.1 on 4 sites I manage.
Reverted to prior backup of my plugins and the site started working again.
Checking error_log files shows that the Class ‘Tribe__PUE__Package_Handler’ is not found in /home/USER/public_html/REDACTED/the-events-calendar/common/src/Tribe/PUE/Checker.php on line 200