Disciplinary Rule 402 is a failure to meet the requirements of the SFA Youth Cup Rule 18. SFA Youth Cup Rule 18 is in regards to teamlines...how they should be presented, when they should be presented, who should be listed on them etc.
I was unable to get my hands on a copy of the Hibs teamlines prior to kick-off on the day of the game, so it may be that. There also seemed to be more than the permitted 6 (not including subs) personnel in the technical area during the game, so it could be that. Or it could be an omission or error in a name, date of birth etc. What it doesn't cover, as far as I can ascertain, is playing an ineligible player as there's a different rule for that. The maximum punishment is £10,000 fine and/or being ordered to replay the match.