Same here. I just finished Roosevelt island, the timer for World Tier 3 was already running when I got disconnected. After logging back in, I saw that the servers didn't log my progress. I was respawned at the entrance of Rossevelt island, still in World Tier 2 with the mission marked as not done yet.
Well, I will certainly not replay this annoying mission. Time to move on to the next game I guess. I don't like those way too long missions with no save points along the way anyways. If I then have to fear that the servers crash at any point, deleting all my progress - no, thanks.
Me and my dad (again) get disconnected when we start mission in co op , same error as you, in other hand my brother play Pay Day 2 and talk on Discord at the same time without getting disconnected on his pc.
And todaty game is unplayable to me its so lagy fps constant drops as hell even sound cut off when i was on headphones this happend second time to me from no reason i guess its related to the network problem.