In this game, I was upgrading my starport and I had to reload from a save game. All of my planetary projects were at 0 or -1 turns to complete. That was unacceptable, so I had to "X" the starport upgrade social project from the list.

Here's the bug ->
The original starport is gone. Clicking on that new empty green space reveals that I can't build a new starport because it is removed from the social improvements build list.

The result is that on the planet screen, it says that there is no starport available, and I can't build ships. But, if I am on the galaxy screen, the starport is listed and I can build ships from there. So, it is not a total loss. As an additional bonus, I'm able to re-use the old starport square for another building.

The memory of the starport lives on!