Secondary monitor switches off when inteldrm switches on

Greetings,

to keep it short:

- older Fujitsu Esprimo PC, Core2Duo, Integrated Intel Graphics
- 2 monitors, connected at VGA and DVI
- during installation both monitors were on all the time.
- Computer switched on, both displays on, boot begins
- inteldrm kicks in, the monitor at the DVI port switches OFF (short message on the display says "power saving mode")
- Xenodm starts, only at 1 display. xrandr reports only 1 monitor, Xorg log says DVI monitor is disconnected
- similar behaviour on FreeBSD, but:
- on Linux both monitors are working with drm and X.

Where to begin to look?

Kind regards,

Henry