I have just encountered the same problem after upgrading to 21. I’m not stuck in maitenance mode, but cron always runs out of memory. For me also commenting out memcache.local' => '\\OC\\Memcache\\APCu', from /var/www/nextcloud/config/config.php, letting the cron job run, and then uncommenting it again seems to have resolved the problem.
EDIT: actually uncommenting again brings the problem back