Every morning I am getting this error message. I wiped the sensor off with a rag, any ideas what caused it. What is the "grid offset"?

166 Y ZERO RET MARGIN TOO SMALL - This alarm will occur if the home/limit switches move or are misadjusted.
It indicates that the zero return position may not be consistent from one zero return to the next. The
encoder Z channel signal must occur between 1/8 and 7/8 revolution of where the home switch releases. This
will not turn the servos off, but will stop the zero return operation. Check grid offset.