Hi and thank you for the excellent description of the issue and the way you answered my requests for info;-)
Unfortunately this is an internal problem with the scanner part of the unit. Usually the belt will have worn or jammed due to a mechanical problem that may have developed over time. This also can happen if there is a power outage or storm in your area that affects the power at the outlet. It may not hurt all of your equipment some are more susceptible to this. And the fact that electronic components by nature of design can fail at any time. look at how much we spend to sent Hubble telescope in space, millions and it broke down shortly after lol. Just to show that these things happen.
I wish I had better news for you I really do but this problem is going to require the service of either a printer repair technician or even a good local home computer repair shop. its technical in nature.
This unit is however definitely worth the cost of repair and my recommendation to you is to call a local copier tech out to fix this as it shouldn't cost much, but they are the only ones with access to the parts and technical documentation needed to fix this.
I will stick with you until this is complete.
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Thank you and have a wonderful weekend!