The bug is persistent with any kind of reboot or reinstall, it's a problem on that savegame.

My system had 4 planets and 1pop, and the birthrate was always stuck at zero.

Thinking it was a food problem, i built every kind of food improvement, but the birthrate was always at zero, with the message "Your current construction is freezing...", the typical message that appears when you have a "colonize" in queue.
I removed everything from the queue, and the message was still present, even with queue empty!

Finally i brought a colony built on another system, forced the population to 2, started a "colonize" in queue and CANCELLED it. Apparently this forced the system to read again the food amount, and now the birthrate is up from 0% to +169% lol.

Backtracking what happened i think the bug triggered like this: having 3 population, adding a colonize in queue, having one or two events in game that killed 2 population and sent the colonize in queue to 999turns, removing the 999 turn colonize.
I cannot exclude having saved and reloaded randomly during these steps.