Situation in short:
White screen instead of expected greeter (screen color depends on selected theme, white for elarun), no errors in any logs, sddm and greeter running. One can actually login by typing blindly and KDE then appears as expected.
Turns out the EDID data for the physical screen dimension shown with "xrandr -q" is wrong, e.g. 4mm x 3mm in my case and the sddm greeter takes this value to calculate its view, resulting in a 15x9 pixel view instead of the proper screen resolution.
One has to create an EDID file and load it into the kernel, overriding the display EDID data, to convince the sddm greeter to create a proper view as there is no other way (radeon) to override the physical display dimension used by the greeter. Setting the "DisplaySize" option for the "Monitor" section of xorg.conf doesn't help.
sddm should not rely on EDID physical display dimensions to be valid and calculate the view size based EDID dimensions or, alternately, needs some kind of override in sddm.conf.
The physical dimensions got probably messed up by an "ADDERlink iPEPS" in my case. This device is OTOH essential for remote management and thus must stay in the hardware path of the affected system.
Creating an EDID file is not exactly trivial and thus no workaround for the majority of users.
This sounds like something you should request upstream https://github.com/sddm/sddm Thanks!