From the TU 9.1 Update patch notes: "Fixed an issue that could cause the doors to not open during the “Break through the Black Tusk Forces” beat in the DARPA Research Lab Main Mission."
Went to do the mission this morning and the doors won't open. I can hear NPCs behind the door.
I just re-ran the mission to capture a video, and of course the door opened properly this time and I was able to finish the mission. I don't know if it makes a difference but when the door failed to open there was a sniper and a warhound in the area. This last time, when the door opened, there was a sniper and a grenadier.