I’m gonna post the same I’d answered in another open post:
“If you have stopped progressing with whatever leader you tried to level up and went on doing quests/pvp/etc, obviously your currently chosen leader will be active and dungeon recognizes that as such hence the error… Make sure you swap back to the leader you were doing the dungeon and you most likely will be good to continue. 
”
This should work. Worked for me, worked for a couple of other members on discord.