Nouveau driver not loaded
After experiencing some X11-crashes, it turns out that X11 was using the modesetting driver instead of Nouveau. For some reason my Debian configuration was not selecting the driver I needed.
If you inspect your X log-file (usually /var/log/Xorg.0.log) and look for Modeline entries, the log lines will start with modeset(0) if you are using the modeset driver, and with NOUVEAU(0) if you are using the Nouveau driver.
To enable the Nouveau driver I had first to make sure the kernel module was loaded. Check with lsmod whether module nouveau is loaded; if not, edit /etc/modules and add nouveau on a single line to load it automatically on boot. Also issue a modprobe nouveau now to load it.
Make sure Debian package xserver-xorg-video-nouveau is installed.
Create directory /etc/X11/xorg.conf.d and add a file 20-nouveau.conf with the content:
Now restart X (usually, logging out is enough).
Instead of modeset(0) lines you should now see NOUVEAU(0) lines in /var/log/Xorg.0.log.
Now I will have to see whether this actually stops my crashes!