Where is the 14th system in the Agrippina subsector?

Solution 1:

It unlocks near the end of the campaign (for imperium at least). After the third to last primary mission, a side mission opens up and reveals the last system.

minor spoiler, the primary mission in question and some further details:

the one where you have to defeat the four chosen of Abaddon. I think it's tied to the one in the Belial sector, but as that was the last one I defeated, it may also be completing the entire mission. However defeating the others also always unlocked a new side mission, some of which revealed previously hidden systems.

You'll find a few more such systems, which unlock after completing a primary mission later in the campaign.