Red is yellow and magenta mixed, so try printing a colour test page, one of red, green and blue, and then cyan, magenta and yellow, and see what the pages come out as. As the line is across the page, and well bounded, that sort of says the problem is a bad wiper on the magenta cartridge, or a blockage on the toner supply, so likely the cartridge, even though it is pretty full, would need replacing.
Pull it out, and place upside down on a towel on a table, then check the drum surface is not showing a defect in that area, and if not then try gently shaking it, and place back in the printer, and try again. If the spot moves there is something inside the drum blocking toner spread. Are you using genuine Xerox toners, or refills, and is it still under warranty on the cartridge, as they normally come with a limited warranty from Xerox for faults in manufacture.
If not that then the main memory in the unit, which is where the page image is stored to read out during printing, has a bad block of memory, and unless this unit uses standard DIMM units, this is a fatal failure.