After updating from PHP 7.0 to 7.3… Getting lots of "Notice: Trying to get property 'uid' of non-object in user_access()"

I have shared cluster hosting and have been using PHP 7.0 just fine. After updating my sites to PHP 7.3, they display the following errors: Warning: session_start(): Cannot find save handler ‘memcached’ – session startup failed in drupal_session_start() (line 287 of /home/hp3-linc7-nfs1-w/483/1780483/user/htdocs/includes/session.inc). Notice: Trying to[…]