I think the sky box gets connected to Decoder so that AV can be set to DVD. My old Avant had the same problem, in that once a Set top box controller was fitted I couldn't access DVD when SKy wass fitted.
Perhaps someone elese has a better idea - it may well be that you need a software upgrade.
Beovision Avant 32 RF, DVD1, Beovision 1, MX4002, Beound 3000, Beolab Penta MKII, Beovox Penta, Beolit 707, Beolink Passive, Beovox C30, Beocom 4, Beogram TX, 4 x Beo4, Form 1 & 2, Beocenter 7700, Beovox S65,