The horizontal encoder is a thin semi-transparent strip that runs across the width of the printer, just in front of the metal carriage rail and helps the print head determine its position in the printer.
We recommend powering the printer off and cleaning it on both sides and along its entire width with some denatured alcohol or IPA and lint free cloth. Below is a photo of its location. You may need to perform this a couple of times to ensure it is completely clean and no residue remains on the strip. You may power the printer back on 10 minutes allowing it time to dry.