Ive seen this is a game breaking bug for many players. Its random for everyone; sometimes its mission 1, 3, 4, and 9. The button locks and it acts like we havent completed the mission. Reinstalling, beating it on different settings, and difficulties, doesnt fix it. I use the pause button on my keyboard during games, but Ive been doing that from mission 1 - 4. The only thing I can think of is I used "-dev" to cheat_revealall on a solo vs pc, I also downloaded a map mod to move camera around; I deleted all that, the only thing Ive yet to do is a full reformat of my PC (I doubt this is on my end anyway). If you are experiencing this GBB= game breaking bug, report it here with your PC specs, and maybe they will figure it out and get back to us.
System Manufacturer: ASUSTeK COMPUTER INC.
System Model: G20CB
Processor: Intel(R) Core(TM) i7-6700 CPU @ 3.40GHz (8 CPUs), ~3.4GHz
Memory: 16384MB RAM
Available OS Memory: 16314MB RAM
Windows Dir: C:\WINDOWS
DirectX Version: DirectX 12
Card name: NVIDIA GeForce GTX 970
Display Memory: 12216 MB
Dedicated Memory: 4059 MB
Shared Memory: 8156 MB
Current Mode: 1824 x 1026 (32 bit) (60Hz)
Driver Version: 188.8.131.5233