iOS 11.3 - Nextcloud can't connect


I just finished my Nextcloud installation on my Synology and I tried to connect with the Nextcloud App.
Unfortunately, there is an error at the very beginning when I am try to connect to the Nextcloud:

I don’t have any idea what this message is about.
Any help is appreciated…

404 points to permission problems on your webserver. Did you set up a redirection to a secure https-connection? Or try a https-url instead of http.

I don’t know how this is solved in the synology app of nextcloud, perhaps ask there as well.