Dirty or damaged encoder strip. You can turn them over because they are two sided. Your service rep should be able to confirm this over the phone and send instructions.
Clean the Linear Encoder. The Linear Encoder is a strip of plastic (appear transparent) which is mounted across the print area above the printhead rail. Don't bend or damage this plastic strip, clean it with a dry tissue, if this won't help with a wet tissue or with a soft cleaning liquid.
If nothing helps the Linear Encoder can be turned. If it is turned until now, and cleaning won't help, you need a new one.