Hi, I have found a solution for the behaviour on my machine with an AMD Radeon graphic card and with regard to Debian Buster 10.0, by doing a search on the Internet.
The following packets need to be installed: firmware-linux firmware-linux-nonfree libdrm-amdgpu1 xserver-xorg-video-amdgpu I found this information at https://linuxconfig.org/how-to-install-the-latest-amd-drivers-on-debian-10-buster Two of these packets were already installed in my Debian Buster installation. Now sddm is shown at the end of the start process of Debian Buster, and I can successfully log into KDE. Regards, Jens
