So it seems like Nextcloud can now act as a OIDC/Oauth2 server (provider?) and let other services authenticate against your Nextcloud user details.
I found three sites mentioning how to set it up:
https://www.claudiuscoenen.de/2018/10/oauth2-with-nextcloud-provider-and-gitlab-client/
https://docs.moodle.org/36/en/OAuth_2_Nextcloud_service
But since I have non of these services I am trying to get it to work with Kanboard, see:
But so far no luck. All I get it a complex error message which I can’t quite wrap my head around 
[index] Error: OCA\OAuth2\Exceptions\ClientNotFoundException: at <<closure>>
0. /var/www/nextcloud/apps/oauth2/lib/Controller/LoginRedirectorController.php line 70
getByIdentifier(null)
1. /var/www/nextcloud/lib/private/AppFramework/Http/Dispatcher.php line 166
authorize(null, null, null)
2. /var/www/nextcloud/lib/private/AppFramework/Http/Dispatcher.php line 99
executeController(OCA\OAuth2\Contr ... {}, "authorize")
3. /var/www/nextcloud/lib/private/AppFramework/App.php line 118
dispatch(OCA\OAuth2\Contr ... {}, "authorize")
4. /var/www/nextcloud/lib/private/AppFramework/Routing/RouteActionHandler.php line 47
main("OCA\\OAuth2\\Co ... r", "authorize", OC\AppFramework\ ... {}, {_route: "oauth2 ... "})
5. <<closure>>
__invoke({_route: "oauth2 ... "})
6. /var/www/nextcloud/lib/private/Route/Router.php line 297
call_user_func(OC\AppFramework\ ... {}, {_route: "oauth2 ... "})
7. /var/www/nextcloud/lib/base.php line 987
match("/apps/oauth2/authorize")
8. /var/www/nextcloud/index.php line 42
handleRequest()
GET /apps/oauth2/authorize?response_type=code&client_id=yrXUHvy6aBqMe9GCdjtv4HJ9dg0lDxlwwQt6m3k19BoJ0orMgAGLr2NChoJzz04Y&redirect_uri=https%3A%2F%2Fkanboard.mydomain.com%2F%3Fcontroller%3DOAuthController%26action%3Dhandler%26plugin%3DOAuth2&scope=&state=f3051ed39be390984f3b89d816c4df5320c999326f6300b19ea8a5689ae3
from IP.IP.IP.IP at 2019-04-10T16:44:51+00:00
Anyone got an idea?