Xerox Phaser 7800 GX color distortion

When printing a few dozen copies of the same sheet, the printed colours changed. The first several sheets were ok but the next ones have distorted colors. From now on, everything is printed incorrectly. In the attachment, on the left an example of a correct printout, on the right a wrong printout (darker), slightly different colors, you can see it on the background of a brown stamp.
Please give me a hint as to what may be the cause of the color distortion.