If you read the Update post, it says only 'natural' 4-star and below characters are avalible to use in these dungeons, so any characters that you got at 5* base cant be used. The devs also said the errors appearing at the end of quests are related to the use of troopers in these battles, so if you want the rewards, dont use any troopers for the time being. There will be a hotfix for this shortly