Boy, tough one, I've not had that problem, and a web search doesn't turn anything up...
Does it happen no matter what interval you choose?
Have you tried completely shutting down the program, start it back up, then change the setting?
What about downloading and clearing the archive?
It looks like your primary software is VWS--I only run Weatherlink, so I can't comment on possible interaction problems...
Maybe there's a communication error of some sort? Maybe you have to shut down all programs except Weatherlink, and try to change the archive?
Maybe there's a way to do it through VWS?
Considering the popularity of tinplate's programs in being able to broker communications between multiple software packages and the Davis console--I'd place my bets on a communication problem...