Nextcloud Permanently Forgets Login

I’m running Nextcloud configured as described in the Arch Wiki (see here) with Nginx and PHP-FPM.

The problem is that the clients (Nextcloud on Android/Linux, Davdroid, Browser) are forced to re-login all the time which is very annoying. I made some changes to the example configuration from the Arch wiki so that PHP keeps a session on the disk (see below). However, that mitigated the issue but it still arises.

user = nextcloud
group = nextcloud
listen = /run/nextcloud/nextcloud.sock
env[PATH] = /usr/local/bin:/usr/bin:/bin
env[TMP] = /tmp

; should be accessible by your web server
listen.owner = http = http

pm = dynamic
pm.max_children = 86
pm.start_servers = 21
pm.min_spare_servers = 21
pm.max_spare_servers = 64

php_value[session.save_handler] = files
php_value[session.save_path]    = /var/lib/nextcloud/session

php_value[opcache.enable] = 1
php_value[opcache.interned_strings_buffer] = 8
php_value[opcache.max_accelerated_files] = 10000
php_value[opcache.memory_consumption] = 128
php_value[opcache.save_comments] = 1
php_value[opcache.revalidate_freq] = 1

Do you have any idea why the clients are forced to re-login all the time?