gentoo-overlay/www-apps/nextcloud/files/php-argon2-en.txt

Migrating from an existing Nextcloud deployment

if you migrate data from a previously existing Nextcloud deployment, you may
need to enable USE=argon2 on dev-lang/php (if previous system ran on a php
version using this hashing algorithm)

This is not listed in ebuild requirements as this will not be required for
most systems (new installs and some migrations)

More details in bug:
https://bugs.gentoo.org/762388