I've noticed a bug I get sometimes, but not always.
Usually, I fend off an attack, I hear the canned response, but if I think immediately try to talk again, I'm able to trade and change equipment.
But sometimes, I fend off an attack, hear the canned response, but then if I try to talk to the settler again, I hear the response again ... forever. That's all they will ever say. Not only that, but if I go talk to another settler at that settlement, I get the same thank you response from everybody, not just the one that gave the quest. And this goes on forever. No more trading with that settlement ever. The only way out seems to be to reload an older save and ignore that quest, if it's on the list, for at least a week of game time. Because I've seen this a couple times, now I am always sure to save right before taking on one of these quests. The first time I had to go back and reload, I lost about 6 hours of gameplay.
So are you seeing this bug, or do you just not want to hear that one line before trading?