After a few turns and for different reasons (changing era in my example, after a city's been occupied then freed as seen in a post from 3 years ago on this forum) the game indicates that no spawn point is available.

It's been a problem for the community since the game released but it doesn't appear in the Known Issue Vauban Update list.

Save and reload won't work. Building a garrison is the only way around the problem afaik.

Would you be so kind as to acknowledge the existence of this major problem in your list of known issues please?

As it happens, the problem appears when you apply the Administrative Centre Renovation.

On the first screenshot, everything was fine. 

Then I clicked on the ACR button and applied it to the city centre. No need to even end the turn.

The second screenshot now displays the spawn issue.