Happens to me as well after the last `apt update && apt upgrade` of my Stretch (amd64) system.
I've tracked it down to be an issue with Slim as switching to an alternate login manager (tried xdm and lxdm) fixes the problem. In the log file, Slim appears to write something about the X server not having been connected in a timely manner. Since the session reset happens in some 10-15 seconds, I'd say Slim has some logic to check whether the X server has become available and if it hasn't Slim retries. Looks like after some late change in either Slim or X.org, this check now fails. If needed, I could try debugging this further or provide futher info (unfortunately, I have no slim.log from the affected machine at hand currently).
