Anton Norup Sørensen (CUO) was contacted about the observed changes in gain, and he agreed that it was worrisome but that the performance appeared normal. He suggested to do some more test before the next time the instrument was scheduled and to change a resistor in the pre-amp of the CCD to get (back) a similar gain to the one for the other CCDs.
No more observing runs are scheduled in the current semester, and it depends when any observing run might be scheduled in the next semester before we decide when to do this.