Unfortunately Dawnguard renders Skyrim unplayable for me.
So far I was unable to get to the Dawnguard fort because the game constantly freezes or crashes to the desktop around Riften.
Crashes always occur at the same locations, when I am trying to leave Riften on one of the roads. For example, I crash whenever I walk down the road towards the bridge leading to Goldenglow estate. Another crash happens when I head towards Merryfair Farm from the main gate. And of course I crash whenever I head towards the fort.
If I disable Dawnguard no crashes occur at all. It seems like the expansion breaks the cell.

To remedy the problem I tried the following things:

Disabled all mods and made a clean save. Did not help.
Reinstalled the game. Did not help.
Deleted all .ini files. Did not help.

Drivers are up to date, stuff like that shouldn't be the problem anyway cause the game works flawlessly if Dawnguard is diabled.

I think it may only work, if I deleted all the save files, reinstalled the game and started a new character without any mods, but I really do not want to do that.

Does anyone know a solution to this problem?

Dawnguard requires 1.6 patch. Have you?

Yes, I am running the 1.6 version.

Where did you download Dawnguard & patches? Steam?

most if not all have to / do O.o

Steam.
I already verified the integrity of game cash but it did not help.
Something is wrong with Dawnguard.
I understand some mods leave their trace in the save files which remain even after they are disabled. Maybe that's what is causing the issue. I don't know if it is possible to clean up save files.

Try 1.7 version. I'm not sure but may be help.