Short story: I need php-apache 7.1 and php-intl 7.1. Unfortunately I have not them in my package cache.
Long story: Nextcloud requires php and php-gd < 7.2. My last pacman -Syu failed to update because it wanted to update these two packages to version 7.2. So I ran "pacman -Su --ignore php,php-gd" and the system upgraded. But I did not notice that php-apache and php-intl were updated to version 7.2 (despite php and php-gd not been updated).
Now nextcloud refuses to start, saying that it is not compatible with php 7.2.