Vertigoth: What worked:
Go and have sex in JoyToy services (you find the spots in map) a minute later i had a Relic crise and the famous Songbird call, and Phantom Liberty finally started.
Sleeping with a joytoy is known to fix or properly activate several game trigger related issues...but I had NO idea that could be the solution to a major game trigger, such as activating the whole Phantom Liberty expansion!
My guess is that if you were at the very end of the base game, there weren't any relevant game triggers left in the game for the expansion pack to hook into and continue. As in, to make the expansion activate itself.
It has been speculated many times earlier that sleeping with a joytoy (during the black screen and loss of player control) is refreshing and verifying the game trigger state, whereas loading or reloading a save game is just reading the game triggers off a checklist, without verifying the trigger state (to save some loading time). And that the joytoy feature was specifically designed this way as an internal troubleshooting method. Personally, I'm inclined to believe that.
But if sleeping with a joytoy was the solution to activating Phantom Liberty...that is another oversight by the game developers.