I think I've found a bug here.

A United Planets vote decided that there would be no more than 2 modules that could be placed on any starbase FOREVER. This happened very early on in the game and I had just 1 or 2 starbases built. Yet I've managed to build at least 5 modules on each one that I now have.

It seemed to ignore the rule for a while and then eventually it decided that when a constructor got to the square just before the starbase I wanted to upgrade, a box opens reminding me of the UP law forbidding more than 2 modules.

Sometimes, however, the constructor manages to land exactly on the starbase square and then I get the warning box. If I ignore it, select the constructor and click Construct Starbase (or whatever it is) I get a box saying Do You Want To Upgrade Starbase so-and-so and away I go.

I'm still playing the game so you can have the log if you wish. The other players are adhering to this law. It's just me that can bypass it.