First trouble was losing one in the woods. It would easily unscrew itself from my Df, and I lost it several times. This time, it probably fell silently into the mud.
While ordering a new one, I tried to re-use the one mounted on my D800. As it was not clean, I washed it with lukewarm water and mild liquid soap. Now it is clean but, lo and behold, water made its way between the two elements - they are not cemented, it seems, or at least not properly. Now it looks like a mild case of cataract...
... sigh ... not sure what to do.