Curious...what was the end result? I have the exact ring on my Dubbel - tasting it for the first time tonight and I noticed the ring. My bottle wand put a little foam on a number while filling and I think that might have been it.
I have seen rings like that before in my bottles and it was not infected. Now, if you get a white film that cracks like a frozen puddle, then you have some sort of infection.