This problem occurs because the dispersion values included in the GRISM static calibration files refer to the current FORS2 CCDs. To reduce data obtained with the old CCD (which had 25 micron instead of 15 micron pixels), you will need to manually override the values for the dispersion [in ц…/pixel] from this table by specifying the --dispersion parameter in the fors_calib recipe. The value for the old CCD is 5/3 of the one in the GRISM_TABLE.