Avdhoeven How much is quite some ? The master frames from MaximDL have a PEDESTAL keyword (offset to prevent negative counts in the files) that needs to be added (if ignored by your reduction software), otherwise the flat might be off by 0.5%, and the bias frame by 10%.
On a sidenote, there might be other reasons, although I'm not sure if they are at play in this case: shutter-time dependent flat-field (which shouldn't be an issue with the electric rolling shutter of the QHY600M on the Veloce but which I have seen before on an FLI PL16803 due to the finite shutter opening/closing time) or a focus dependent flat-field (this effect is hopefully small if the flats were taken with ~ same focus setting).
