I went back up and looked again at what I could see of your planter and I'm pretty sure it's your problem. It might be tall enough and it might be long enough but it doesn't look to have the width nor the depth that your mature wisteria needs.
This is what I would do:
If you can dig a round hole that is at least the circumference of the long opening in your planter and as deep as your planter is - do that. If your planter is 4' long, then dig a 4 foot plus round hole that is just a little deeper than the planter. It wouldn't have to be exact but shouldn't be smaller than.
Loosen the soil in the hole and make sure it's very good, with good drainage and rich enough to feed the plant.
If you can remove the planter from around the tree so that the roots are not terribly disturbed - do that.
Then, if you can use a water hose and gently rinse the old dirt as much as you can away from those roots, do.
Then lift the entire tree including the root ball and place it in the hole then gently but firmly press the soil all around and into the crevices of the roots till it's completely and compactly planted.
If you can do that, I think the tree will be just fine with frequent watering through the summer.
I had already read that first link that Sue listed above and that sounds difficult and requires root pruning - not always a good thing with a wisteria's root ball; so what I'm saying is something a little less intensive and invasive to the roots, but it's what I've done with an up to 5 or 6 year old tree or two. Haven't lost one yet. However, what I was sure to do was not disturb the root ball. I did cut some side runners back, that won't hurt it, but I never disturbed the root ball, just washed it off, gently, not with a power washer or a hard spray.
Now if you can't remove the sides of the planter and get it out, then I'm not sure. With a mature wisteria, it's important to just loosen the soil within the root structure but disturb it as little as possible. Yanking it out would no doubt give it hysterics or something. I'm not sure how to get it out without removing the sides but you probably can figure it out.
It's a risk, but I think you will lose it anyway if it remains in that container. If you can get it out and transplant it gently, I think it will be just fine. I wish I were there to help you because I know how it is to watch a much loved plant wither away. Heart breaking!
Worth trying, I think.
