Solved, if with a workaround. I used the console command setstage to move the quest at various points. Setting it to 0 didn't work: still no quest from Sadri, and I still had the ring. Setting it to 10 (start) put the Beast ring in a lair, when I gathered it, I still had 1 of this ring in my inventory. I got the other ring, but the "return to Sadri" stage didn't come up, I had to use setstage again (both rings gathered). At this point, I had the Ring of the Beast still uncompleted (even if i entered setstage for that!), and the other ring's subquest was accomplished. When I got back to Sadri, the quest completed, and I could ask for new ones. That was messy...