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.