Is there a "universal" problem playing Dual Discs?

DualDisc is not quite dead yet.

Just a slight correction, there are no problems with playing SACD's on any universal or other SACD capable unit. Only DualDisc's can have a problem. Besides that issue, DualDisc is now a dead format.

I have never had a problem playing dual discs in my 5910CI. Make sure you set the player for audio in the Denon setup menu. Otherwise it will only playback the DTS layer of the disc. A word of caution though some DVDA discs will not show video through the HDMI output when set to the audio mode. If this occurs with the Denon player you may need to hook up and alternate video output for DVDA playing.

I believe the UDP-1 uses a Pioneer transport. Can't recall which one, though. I've sent some emails to McCormack asking, but they've not been too quick to reply.

My Esoteric DV-50 played them fine [same transport as the UDP-1 I believe], but the Denon 5910 does not play them...which might be true accross the board for the Denon players.

Dual Discs are slightly thicker than other discs. As a result, some players are unable to read them; the laser just doesn't line up correctly because of the extra couple millimeters.

If I were you, I'd contact the manufacturer and see if they've had success or failure playing Dual Discs in their machine.