My expertise with back pain comes from dealing with my own back pain issues which may not apply to your problem. Part of my solution has been to be very careful with any lifting and to maintain the back straight and to lift with the legs. The other part of my solution is to work with some weights to maintain strong back muscles. As your back pain does not appear on an exercise bike that is more upright, your issue seems to be related to a bent back. This may not work for you but, on my road bike, rather than bending at the waist to reach the handle bar, I think of folding at the hip. This focus tends to keep the back straight. Best wishes.