ha, got no email informing me of your reply, sorry for the delay..
1) yeah, i figured that out by testing. it is not related to the Roland but to all external displays in fullHD or lower. once i switch off windows scaling and set my notebook screen to half its native resolution, suddenly secondary display is detected correctly. it's a work-around, but quite an annoying procedure, plus i loose the benefits of my QHD screen.
is there any update of Arena foreseeable that will take care of hiDPI issues? maybe even include an adjusted UI (no upscaling, rather bigger/sharper icons and fonts)? i know that is a lot to ask for ..
2) found a way to use it. i have to start midiOx first, connect with the nanokeys. then stop midiOx and start Arena - voila, Nanokeys are present and working. i guess the problem are bad Korg drivers, as both the Nanokeys as well as the Nanokontrols sometimes disappear and reappear when changing midi software. so i sometimes end up with a list of midi devices: Nanokeys1, Nanokeys2, Nanokeys3 ... only the latest working.