Currently X will start again. As it seems, the modules for nVidia needed kernel headers to compile, which were not available for kernel 4.9.0-3. Changing kernel and header by hand to 4.11.0-1-amd64 and forcing a recompile of the nVidia kernel modules solved the issue -- at least temporarily.
I think it is very important that, on error, the X server tries to boot into a default X server that works for all graphics adapters (vesa ... ?) so you get a graphical user interface at all. Text mode as fallback, especially in 80x24, is a little heavy-handed for all those inexperienced users out there.
