At a certain point in the "Underground Undercover" quest, Z1-14 asks you to kill some synth guards. But if you try to actually do so, when you kill the first guard the Institute becomes hostile. Attacking from hiding doesn't help. This causes Underground Undercover to be marked as failed, as well as numerous other Institute-related quests.
I'm sure I've successfully completed the Institute ending on previous playthroughs, which would require completing this quest. So I'm not sure what's different now. But I just tried about half a dozen times with the same results. The only mods I have installed other than the unofficial patch are one which restores XP for Sandman kills and one which fixes the requirements to build crafting stations; neither seems likely to be the culprit.