If you can send me a save before the quest it could be helpful! The saves are located in the saves/ subfolder in the installation directory.
I didn't have a save from before the quest, so I went through the game again and made a save beforehand. I went through the quest again, just to make sure it still wasn't working. Sure enough it still won't work. I have the save files for before the Three Oases quest, during it, and after I "finished" it, but it wouldn't register as completed. How do I send you the save? I can't attach files here. And do you want all three saves? Or just the one from before the quest started?