Climb1, I also suffer from this problem with the filter.
I have other issues with the smooth running of RadarBox, which have been aired in this forum a number of times, but in respect of which I don't seem to be getting anywhere. I've been in contact with Andre Brandao about these issues, including the one which you detail (quote from my E mail: "I sit and wait for ages for menus to appear, or clicked buttons to function. I've also found that selecting the Airline filter menu rarely works - either by selecting All or a specific airline. I am sitting here at the moment staring at an empty radar map."
I appreciate that mine is allegedly an unusual case (as Airnav is at pains to remind me), but promises of possible remedies have yet to be fulfilled.
DC