Do vertical cracks in old plaster wall along studs indicate a structural problem
The pocket door that is dragging the floor is the best indicator perhaps of what is going on. If it is original to the house means one thing, if it is a newer install as in not original means something different.

If painting the wall covers the crack you are asking about, tells me they are very small and there is most likely no concern. Old houses move, Plaster walls don't like to move and when they do, you will get settlement cracks, which it sounds like what you have here. The again if the pocket door is an add on or part of a remodel, that will affect the wall. Still no concern. The door that is dragging the can be from a few possibilities. The header supporting the track failed. I don't think so, your wall you asked about would have worse cracks. The track may have become detached from the header and the door dropped. (poor workmanship)

It may be good to post another question with pictures about the pocket door. That appears to me to be more of an issue than the cracks in the wall.