I "finished" the Nuclear Option for the Minutemen, I need only the "Talk to Preston" point, but he doesn't seem to "accept" my answer. Every time I init the talk, it starts over again, again and again, no matter what option I choose.

I choose no evacuation orders. Also at first I said no to Shaun and then I changed my mind, maybe that triggered the bug? Because he is not at the rooftop.

Can confirm now that my previous assumption to leave Shaun behind and then take him with me didn't trigger the bug. I reloaded the savegame prior to teleporting out of the nuclear reactor. This time I told Shaun to come with me. Sturges teleported Preston and me to the Mass Fusion Executive site: I triggered the button. Ending shows. I get the instruction to talk to Preston, I do, but the instruction does not change and Preston has the same dialog as before. I can't complete the quest. (Although the achievement was triggered) The quest stays active in my quest log.

This guy had the same problem, but he seems to be banned: https://steamcommunity.com/app/377160/discussions/0/496881136927342840/