Philipp Kern <pk**@de**.org> suggested (on the Debian bug tracker) changes to the current screensaver detection code:

frontend/common/src/System.cc:
  if (is_kde() && (lock = g_find_program_in_path("kdesktop_lock")))
    {
      xlock = g_strdup_printf("%s --display \"%s\" --forcelock",
                              lock, display);
    }
  else if ((lock = g_find_program_in_path("xscreensaver-command")))
    {
      xlock = g_strdup_printf("%s --display \"%s\" -lock",
                              lock, display);
    }
  else if ((lock = g_find_program_in_path("xlock")))
    {
      xlock = g_strdup_printf("%s -display \"%s\"",
                              lock, display);
    }
  else if ((lock = g_find_program_in_path("gnome-screensaver-command")))
    {
      xlock = g_strdup_printf("%s --lock", lock);
    }

This works correctly for KDE users as `kdesktop_lock' is invoked. But it
does not work for Gnome users when xscreensaver is installed [but not used].
`xscreensaver-command' does just bail out with the failure that no
xscreensaver is active on the current desktop[1]. No further xlock command
is tried, because only one is set within System::init(...).

IMHO the order should be changed so that xlock is last (as a fallback if
nothing else works) and the various commands should be tried in order
until the first exit code is zero.

Created attachment 102
Locking fix

Philipp Kern has proposed the following patch which fixes the problem on his machine.