No idea, that's just what I read on internet. I'll send a mail.
EDIT: Sent it, now you better remove your addy from here. ;-)
OK, I've tried it, and I can confirm this bug is
in the GOG version as well. What is especially strange is that the berries don't even show up in the quest items list, but the pointer points clearly to the witch.
So yeah, this bug is still in there.
EDIT: I've just tried it with an own character, with "tabula rasa" (created a LAN game just for myself and started a hack'n'slash session, thus no quests are done), went to the oasis, completed the quest about the correct urn, then went to the witch, got quest, picked up goldberry, but it didn't show up on my char's Quest items list ("Key" was the only entry there), and the witch didn't finish my quest. So, same results
Conclusion: only buy GOG's version if the convenience of no CD required even for installation and the extras are enough of an incentive to do so.
Alright, after reading some posts on some other forums, I've come to the conclusion that there is a bug with The Riddle of the Desert quest, which breaks 3 or 4 other quests (Whispers in the Wind - the one we're talking about here, Where is the Blacksmith, Painting in Mascarell, and the last stage of The Unknown Warrior). This is a known bug, and the recommendations are to leave The Riddle in the Desert quest till you've done these other quests, or skip either The Riddle in the Desert, or any of the other quests you did not do before doing that one.
I, myself, am going back to before The Riddle in the Desert (which is a long way to go back, for me), and trying to finish the others, then come back.
As for the key, as I recall, mention is made when you get it of a place where it is to be used (that, or it's in the quest listing for The Riddle in the Desert, after you successfully find the key).
[Note: I usually try to do all the quests in an area I've been sent to, seems I need to be careful not to do that if I was only sent there by a side quest. On the other hand, I don't want to complete the main quest line only to find that there are a lot of places I never went to and early quests I never solved, because I just followed the main quest line and only did quests in those cities.]