My interpretation of the rules was: Ranged attack - target needs to be on the same path as the attacker, melee attack - target needs to be adjacent to the attacker, but may be out-of-path (so no line of sight). After there was quite a discussion on this forum and several people claimed you needed line of sight to attack in melee, I sent the question to FFG. While I seperated ranged and hand-to-hand combat in my question, the answer was: "You can always attack adjacent characters, even if out-of-path." The answer neither mentioned ranged nor hand-to-hand combat. As we say in Germany: "Alle Klarheiten beseitigt!"
Sorry, I can't really translate it, it means that the answer only made matters harder to understand and caused confusion. Where's a new FAQ to explain all this?
Played with my buddy the first time last night. He got that even though 2 circles were adjacent but no shared path, he couldn't fire through the door (tropper wouldn't know the stosstruppen was there and thus wouldn't shoot...or doors were made of some advanced nano fog ...or u-world stuff ). He then verified - "and you can't attack me either, right?" "well, actually I CAN - H2H weapon on my stosstruppen can ALWAYS attack an adjacent circle, whether it shares a path or not..." "....umm what... [i point out in rules, forums] ... ok they have to be nano-fog to stop only bullets." we laughed and moved on playing per rulebook.
then, and now seeing FFG's answer, the German phrase that comes to my mind is "Mensch ärgere dich nicht!"