gdmflexiserver goes crazy

[Hendrik Boom <hendrik@topoi.pooq.com>]

Recently, when I run gdmflexiserver in order to get a new login screen, 
the screen goes black and stays that way, with the monitor's little green 
light near the on-off switch blinking as if there's no signal.  Control-
alt-F7 gets me back to the existing screen (with the terminal on which I 
typed the command), but it takes a few seconds for it to fade in, as if 
it's just powering up again.

Running a Debian testing system, updated every few weeks, occasionally 
oftener.  I normally use icewm.

Any idea what to do about this?  Where to look for clues?

-- hendrik