MacOS - unable to connect to desktop-client 3.0.3

Hi there,

after installing Nextcloud (20.0.1) on a hosted server (https:// - IONOS), everything is fine via browser.

When trying to connect the desktop-client (3.0.3 - which works perfect with a second nextcloud-installation on a different IONOS-server), I am asked to log in via my browser and grant access what I “successfully” do. My browser indicates “access granted” afterwards, but the client complains that the server does not allow access and asks me to grant access via my browser… .

Same with my Android-Client which gives out a 401 http-mistake (the Mac-thing does not give out anything).

What can I do?

Thank You so much for Your help,