Okay, I'm playing the Dawnguard DLC with the vampire path and I'm at this quest called "Unseen visions". Basically I have to find a Moth Priest's knife, collect some butterflies and then talk to Serana to continue the quest. The problem comes from Serana that is somehow glitched and refuses to follow me whatever I do. On top of that, when I take the Moth Priest's knife the chamber in which I'm in and Serana is supposed to walk in gets auto-locked by an unpickable door. So Serana is stuck at the other side and I can't interact with her. I tried killing her and reloading last save, fast travelling and so on... Nothing helps, she justs stands at the beginning of the dungeon and refuses to follow me even if I try to give her a Fusrodah boost. On the other hand, when I select the "We must part ways" option, she refuses to abandon me and continues to stand in one place. Earlier autosaves are a bit overwritten by my tries to debug her.
 
Any ideas on hwo to fix this? Any script that would unstuck her or whatever?

Try standing next to her and pressing ` <- button next to 1 to bring up the console. Click on her and you'll get her editor ID along the bottom. Type Kill and close console. She might or might not die. Now bring the console up again and she should still be selected and type Resurrect .

Failing that you can try unlocking the unlockable door by clicking on the door and typing Unlock however this might not work if a number of things have to happen for the door to open.

Another option is to disable ALL mod's you are running so the only things that the data manager should have ticks next to are skyrim, the update, dawnguard and the hd pack.