What I can see seems to make your guess correct. The glass is definitely split, maybe by load, but the freezing option seems very likely. The backer looks newer, not original, and the bolts down low were added. This is not structurally as strong as higher up, so it seems they were grabbing what they could to transfer some of the load. I have not rebuilt a knee, so others should define some of your options, but seem clear you will have to cut open, replace and reglass. Bummer
I do a lot of construction jobs redoing what some other guy did before. Many fix the damage, or symptom, but fail to properly identify the cause. You will want to figure out where the water
comes from and make sure you solve that problem(s) before you close it up again.