I reloaded a game before entering the castle, entered, went and talked to Ben, then got the bottle of ale from the room just north of the throne room (
screenshot), and returned to complete the quest. If I complete Janus' tasks and get kicked out of the castle before completing the quest, I can still give Ben the ale from outside, but that doesn't change the failed quest entry and there are no experience points given.
I had no other alcohol in my inventory entering the castle, and only picked up the one bottle, so that may have helped. Too much alcohol or a cluttered inventory may be part of why a few people have had a problem with this quest.
I updated the
Lost Items post with the inventory files to replace the bottle of ale, if you still have a save in the castle really want to complete this quest.