That was frightening to see. I would guess she has something neurological going on. It would be hard enough trying to diagnose in a human let alone a chicken. But some seizures in poultry are caused by vitamin deficiencies, so that's definitely something you can do something about.
I would continue with the vitamin E and give it with a sliver of selenium for maximum absorption. Try giving it right before she goes to roost for the night so it's at its maximum strength in her system when she can be expected to have a seizure.
Continue with the B-complex. Deficiencies in some B-vitamins can cause seizures. Also, start giving a calcium supplement with magnesium. Deficiencies in those can cause seizures.
Ideally, we would only treat with one vitamin at a time so we would know which one worked should the seizures stop, but if it was my hen going through this ordeal, I would do all of the vitamins just on the chance one will work.
If the vitamins do work, after a couple of weeks, you could decide to withdraw them one at a time to see if the seizures start up again. Then you might know which vitamin was responsible and continue with that one.