Thanks for the reply. The HannsG is used as an extra monitor and it is the only one I have. The mac-mini is normally connected to a Samsung TV via an HDMI to HDMI cable. I sometimes connect the HannsG to my macbook pro as a mirror monitor via a DVI to DVI cable. In both cases the monitor works well.
When I try to connect the monitor to the mac-mini via an HDMI to DVI cable or a Thunderbolt to DVI cable no image is shown and the monitor goes to sleep.
The same goes for the HDMI to DVI adapter using the same DVI to DVI cable for the macbook pro.
If I connect the monitor to the mac-mini via a Thunderbolt to VGA cable then the monitor works. So it was suggested that it is the DVI socket that does not work. But the DVI socket does work when connected to the mac-book pro.
I will look into links you have suggested. Thanks again.