Sorry my English is very bad :( I hope you understand me.
The first time you travelled without Serana. It activated the event dialog.
The second time you travelled with Serena. The event dialog was activate. you don't will have dialog with Serana.
Solutions without commands.
1.-You leave to castle of Volkihar.
2.-You travell to any town.
3.-You save the game or progress.
4.-You leave the game and execute to skyrim launcher. choose datafiles and desactive DLC danwguard.
5,-You enter de game and load your game or progress.You lose objects (armor vampire of Volkihar that you get or crossbow of guard of Alba.) and points of maps the dawnguard. You don't will lose anything.
6.-You save de game again and leave de game.
7.-You execute skyrim launcher and activate DLC dawnguard.
8.-Enter de game and load you game. Now you repeat de mision of dawnguard and the bug finishs.