Start with the floor exercise to warm up. Get up to about 20-30 reps. On the roman-chair, start fully down and lift up to 45-degrees in sets of 10-25. Once those are easy, go all the way up to 90-degrees (relative to floor). Again, sets of 10-25. Then start hanging onto weights 5->10->30lbs should do.

It's a matter of technique then. There are plenty of people that get fitness improvements with weight workouts in the gym. From way back to ancient Greek times and the beginning of the Olympics (and earlier), people have employed weights and exercise machines to increase their fitness. Sure, there's are also a small percentage people who get injured as well. Probably due to a pre-existing condition, incomplete recovery from injury or improper technique. Going in and trying to squat 500-lbs on your 2nd session will most likely cause problems.

Similarly, if you've got a sore back, those muscles are injured. Care must be taken for as complete recovery as possible, then adequate warm-up and stretching. And the exercises themselves should be done at little effort initially to develop proper form and motion. On the roman-chair, this could be going from full hang only up to 45-degrees or so. Only after sufficient "base" training has been done do you want to increase the resistance. I suspect you may have overdone it your 1st time back. A personal-trainer comes in handy here.