I think you might need one of those floppy cables with a twist in it, was something on Amibay about it, anyway hope the info helps you get to the bottom of it 
Steve.
Thanks for the hint, but the cable is fine. It has the correct twist.
Meanwhile, I got it working, just formatted a HD disk and it shows up as 1760K drive now.
Thanks for the help guys (A4000 Mad, pan1k, fitzsteve) :-)
The aspect I wasn't aware of is that the drive is configured as a 880K drive during boot, and shows up as 880K drive in the format tool (right after boot). But only after a HD floppy has been inserted it switches to 1760K mode (which can be observed live in the format tool). After that it remains in 1760K mode. I had thought it to be the other way round.
Secondly, the drive is not always performing the mode switch, sometimes it fails to do so, and this probably fooled me sometimes.