Unable to get session by Ctrl-Alt-F<n> from Xfce, fvwm, fvwm3, (etc?).

Ctrl-Alt-F<n>, n=1-8, when the xdm login dialogue, or whichever of Xfce, fvwm, and fvwm3, is used after login gives a blank screen on my Dell D630 laptop.

The only useful actions found so far are Alt-F9 to get back to the xdm dialogue, or the Xfce, fvwm, or fvwm3, session; or Ctrl-Alt-Del to reboot.  The console log is visible with the xdm dialogue and a 'Timeout initializing vt_vga' message appears after each return to the X session by Alt-F9.  Same for fvwm3 session; did not check while fvwm was installed; do not know how to display console log in Xfce.  Probably not hardware problem because removing the FreeBSD installation from the modular bay and replacing it with an OpenBSD installation gives the expected behaviour: a visible login prompt after Ctrl-Alt-F<n>.

The xdm login dialogue looks rather large, 95% at least width of screen, probably related to the 'No image conversion tools found, using original icon size instead.' message that is in the ~/.xsession-errors file.  I see no hints of what should have been installed to fix this.

What settings are needed, or perhaps what needs to be installed, to fix the problem with Ctrl-Alt-F<n>?

Roger