Being a solo player I rely heavily on the AI partner feature. Well, every now and then I'll have an error pop up saying I can't "invite" another AI to my team because it's "full." A full team is 4 (myself and 3 others, whether AI or players). I've had it pop up after inviting two AI, I just had it pop up after inviting only one, and it does this for both NPCs and "Free Players." I've also had it refuse to bring an NPC in because "This partner is in use by another character."
Sometimes I can get around this by "booting" all the NPCs, sometimes by leaving the "gate ship" and goign back to the main lobby. But not always. And it's really annoying and time-consuming having to fiddle around trying to get it to work again.