Tumors along the spinal cord might cause seizures. There are chicken viruses that cause these tumors, and it can also lower the chicken's immune responses, making a thing like sour crop a chronic issue.
If this hen should die, I recommend you locate a lab to send the body for a necropsy. It will tell you if you have one of these viruses in your flock. The other birds can be well and still carry it.
Meanwhile, all you can do is treat the symptoms. For sour crop, the treatment is miconazole twice a day for seven days. It's an anti-yeast medication found in the pharmacy where feminine hygeine products are sold.
Your hen may also have had bouts of impacted crop. That's when a hard mass cannot be processed in the crop and blocks new food from going through. The treatment is oil. I like coconut oil in the solid form because it's easy to slip into the beak. I use one or two teaspoons to start, massage, use another teaspoon an hour later, followed by a stool softener. Massage after you've allowed it to work for an hour and the contents should empty.
For the seizures, cannabis, if it's legal where you are, can help. B-complex vitamins may help, along with vitamin E 400iu, and a small amount of selenium.
