I can confirm the above described issue for:
Nextcloud 21.0.3 (after upgrade from 21.0.2)
Debian 10.10 Buster
Apache 2.4
PHP 7.3.27
Thanks to @bb77 for the explain and reminding one to heed the admin settings. However, this did NOT fix my issue. All settings in place.
On my machine there is a condition not mentioned above:
$ sudo -u www-data php /var/www/nextcloud/occ maintenance:repair
An unhandled exception has been thrown:
OC\HintException: [0]: Memcache \OC\Memcache\APCu not available for local cache (Is the matching PHP module installed and enabled?)
No such issue in any version before 21.0.3.