I took the time to reproduce the issue by installating both Fluxbox and Sugar and restarting the system. On gdm login screen, the session is available containing Fluxbox and Sugar.
I do not know how you cannot access to Fluxbox while I successful got there without a fuss.
You missed what I said. It did appear, but it needed a reboot first. Anyway, I would much rather just be able to have logged out.