Probably mentioned somewhere, but I couldn't get Search to work... and let's be honest: I am lazy.

The UN Planets resolution decreed that no Starbases can have more than x-amount of mods. This is enforced when you just send your constructor to it: a window pops up telling you're not allowed to add more when at the max.

However, if your constructor is in the same space as the starbase and you select it, tell it to improve the base. Tadah, no sweat and welcome to the upgrade window and upgrade all you like, no matter the UN resolution.

Shouldn't be working like that I think