TLDR;
run this from command line to get nextcloud 25 working on 32bit systems.
sed -i 's/self::SIZE_PROPERTYNAME, function () use ($node): ?int/self::SIZE_PROPERTYNAME, function () use ($node)/' /var/www/nextcloud/apps/dav/lib/Connector/Sabre/FilesPlugin.php
There are workarounds available, see
https://dietpi.com/forum/t/nextcloud-25-on-pi3b-this-directory-is-unavailable-please-check-the-logs-or-contact-the-administrator/14996/3?u=jappe
and
https://dietpi.com/forum/t/nextcloud-issue-after-updating-no-longer-showing-files/14960/2