Tin in solution can fog film, but I haven't any idea how you'd get that in Df96 -- even if you mixed or stored it in a soldered container, there's nothing in the monobath that could pull the metallic tin from the solder into solution. Sulfur compounds can fog halide, as can ammonium ion, but the fixer component in Df96 (at least the more economical and more storable dry-shipped version) has little ammonium in it if any -- I could see the sodium thiosulfate crystals in one bag, and even if there was ammonium chloride also in the mix, there isn't enough ammonium there to have much fogging effect (as witness all the films processed by Huss and many others in this stuff without issue).
Overall, the simplest and most likely explanation for film that was "fogged by Df96" is that the film was fogged in handling, most likely either during bulk loading (if that was done) or in loading it into the developing tank.