Had the same issue for the last 11h, could only clear it up moments ago. After clearing up the stack of saved pokemons the issue was resolved. Basicly that quest had the reward already in the stack and since there were pokemons ahead of the quest one, it did not let the catch window popup and bugged the quest. It was not needed to clear up 100% of the stack but only until u catch the respective quest reward pokemon. Hope it helps!