In The Netherlands we've been seeing the white ring for some time now. We don't know what causes it, but it has nothing to do with using your bottle for too long a time. It seems to increase with time, but we've seen the white ring in completely new bottles. Intervet (Vetsulin/Caninsulin produces) has told us to return bottles with a white ring to the vet, to exchange for an new one, as they cannot guarantee the quality of the product when a white ring shows. However, more than one of us, when returning the bottle to the vet, found out that sometimes a whole batch seems to be affected. This happened to me. When going back with my bottle to the vet, he got out everything he had, and all bottles were affected and had to be returned.
Over the last couple of weeks complaints in de Dutch feline diabetes forum have increased that not only the bottles show white rings but also that the fluid shows little blobs and thread-like stuff, like thin worms.
Intervet keeps denying that there's something wrong (if they react at all), but at the same time advises to exchange affected bottles for a new one, at no cost.