Creating new followers (  Aja and Kaida  ) I got the "Serana" bug at them. 
Following this up, I found the reason triggering this bug. Fixed this for my followers
and on this occasion the Serana bug, too.
Bring the mod very low in load order and via console "disable" return "enable" Serana. 
Do not close console between this steps. Close console, target Serana, open console again
and enter "resetai" to have changes applied. Best save and reload then.

V3
new feature, switch to default sandboxing via ring. Wear ring, default sandbox, no ring Serana sandbox.
Check, if she wears the ring. 
Ring craftable, 1 silver ingot, one ruby at forge. See picture for name of ring.

V3.2
Serana original Sandbox. Do whatever you want, cell change fight, sneak, not sneak.
There is one IMPORTANT point. Serana will remain in the state you have when killing her victim.
Come out of sneak before she kills her oponent. This is due to the Serana behaviour, I cannot change this.

V5.0
fixed stay in fight (need feedback here, as I don't have this bug)
Found something odd here. The 2 bugs are related to the same culprit.

If she stays and doesn't move, run away, 50ft should be ok, or out of LOS and if she still doesn't follow, go in sneak and out of sneak.
Now she should walk normal again. 50ft is estimated, this number is defined in her script. Will not check this.
If a mod removed her live drain, you need to give it back to her. This is part of the fix. Original as in Dawnguard defined is important !
Let overwrite EVERYTHING, yes this one, too.

V5.1 spell trigger exit added.
V5.2 spell tweaked to show reaction

Added optional file
V1 
Serana uses staffs.
2 staffs, 2 daggers. She will dual wield with daggers.
craftable, FE flagged