To make matters worse, the debug version I created and compiled, works. Apparently there is a difference between the igmpproxy-0.2.1 in the freebsd repository (I didn't build that) and the same version when built with freebsd ports. So I was able to build an instance of 0.2.1 that works, but I'll never know what is wrong with those others.

Many thanks, Steve! Really appreciate your help. Kind regards Phill