I too had the problem today. The error is from cronjobs that run
/usr/bin/php -f /var/www/nextcloud/cron.php:
This version of Nextcloud is not compatible with > PHP 7.3.<br/>You are currently running 7.4.2.
PHP7.4 is installed through sury:
Maintainer: Debian PHP Maintainers <email@example.com>
Installed-Size: 70.7 kB
Depends: libapache2-mod-php7.4 | php7.4-fpm | php7.4-cgi, php7.4-common
Download-Size: 35.2 kB
APT-Sources: https://packages.sury.org/php stretch/main amd64 Packages
Instead of deleting php7.4 you can set php7.0 as default and everything works again:
update-alternatives --config php
Where sury comes from - I don’t really know.