The sensor is dirty.
Switch the robot vacuum cleaner off. The switch is located on the side of the robot vacuum cleaner. Depending on the model, press the I/0 switch to switch position 0 or the ON/OFF switch to switch position OFF.
Place the robot vacuum cleaner upside down on a level, clean surface.
Clean the sensors carefully using a dry cotton wool bud (see right half of the image).
RX1: Also clean the sensors on the side of the robot vacuum cleaner (see left half of the figure).