Perhaps it's just been bad luck, but the only three alien races I've come across have attacked and destroyed my unarmed exploration vessels immediately, and went on to be impossible to communicate with.
I always put together a diplomatic team of between 120 and 160 combined Communication points, and the first five or so rounds of communication attempts tend to have minimally positive results. And then things go south, and get worse and worse until they give up.
Is communication failing because they attacked me? Is the negative modifier applied for the violence affecting the rolls? Is it impossible to overcome a diplomatic penalty from a first contact attack? Or is the fact that they immediately went hostile caused by the same 'trait' that is making them impossible to communicate with?
I feel like attacking a strange alien ship in their system out of a sense of paranoia/defensiveness shouldn't necessarily mean that the aliens are impossible to communicate with. If I leave their system and try to communicate, I feel like it should be possible to bridge the gap and put the lost ship down to a bad first contact.
I also am curious what exactly 'Impossible to communicate with' actually means, and whether it is a permanent thing. Is it saying that communication is literally not possible or is it saying that the aliens have no interest in communicating? Is that status permanent? If it is, then is there some way of ensuring that the communication attempt doesn't go off the rails, beyond putting together better teams?