I installed SeaHorse: under Passwords/Standard keychain there are only Skype entries. Nextcloud I cannot find in any section.
The client version is 2.6.2-1.1
The server is self-hosted at a Raspi in my intranet, current version.
Ok, Solution:
Gnome-keyring was installed, but not libgnome-keyring.
Installing the latter fixed the problem. Now Nextcloud shows up in Seahorse.
Thanks very much 