User cannot login in Nextcloud embed in iFrame


I have a problem with Nextcloud embad in iFrame.
If I try to login as a normal user I get the message "Zugriff verweigert - Logged in user must be an admin"
As a user with admin-rights I can log in.

When I first login as a admin user and log out and login as a user without admin-rights I can login.
This is happing on all Browsers on a Apple Device and in Windows IE. Windows Firefox and Chrome works.

Any ideas?

They have a same-origin policy in the headers, so you can’t use this within (i)frames. You could disable this manually but it was placed there for security reasons: