Yeah, the gait hiccup sounds like a belt issue to me, too, given the long storage and all.

I've found treadmills to just be hell on my back to the extent that I don't use them anymore, but I also have a lot of muscle deterioration in my core because I have shortened tendons in my ankle that makes me walk like a toddler in high heels.

So I can confirm that most back and especially lower back/hip pain in regards to treadmills comes from having under-developed muscles in that area that are being pushed past their limits, which is making the larger and more developed muscles overcompensate.

So, I second all of these suggestions. Check the belt tightness, keep a close eye on your posture, and go slow until it stops hurting.