Just to add to this, I also tried completing this quest line today and had the same issue. Unable to progress through the Mushroom Shrine due to this bug.
Upon entering the Fox Financial Area to kill the Yakuza I have about 5 seconds before I disconnect and get the same error "Unable to connect to Game Server, Please try later."
Character Name: Riyek
Character Level: 203
Character Job: Blaster
World Name: Reboot
Date and Time: 19th December 2018. Particularly between 1 and 2 pm AEDT and then intermittently since then (I've been trying repeatedly to brute force the quest and nuke them all before I disconnect).
**Update: I was able to finish the quest last night. I managed to nuke all the Yakuza within 3-4 seconds and end the quest before DC'ing. Once the quest completed, everything carried on as normal. I did not DC once the quest completed, and stayed in the map for the follow-up scenes.*