It was a glitch last year as well and default uniforms/playbooks have nothing to do with it. It is pretty much pre-determined to happen and there really isn't anything you can do from inside the dynasty itself. The work-around that worked best for my OD last year was have one of the two National Championship teams save the dynasty file and create a separate OD from that saved dynasty file. It should load up right at the NC game. Then invite the opponent and play the game in that dynasty. All the settings, rosters, etc will be identical. It should load just fine. Then play it out and see who wins. After that, just go back to the original, have the commissioner set the loser to autopilot and the winner can just play and win the National Championship to get the trophy and continue the dynasty. The stats and scores won't matchup in the two games, but it's the only way I found to work around it.