I'm in the same boat. I just finished the quest. Sai Sahan told me about a new problem, a letter from Tharn and told me to speak with the big red dragon. He/she/it got a quest marker but I cannot interact with he/she/it.
EDIT
After a few login/logout, I managed to interact with the dragon. I suspect there's some problem when the game forces you do wear the mask and hide costumes and hats. I guess it also changes the cam angle and messes up, the interaction focus is not the best thing in this engine anyway.
Edited by jeanbulinckxub17_ESO on January 19, 2020 8:08PM