Michiel Visser wrote:
Have you recently changed a light bulb in the room in which you work?

There is no light bulb turned on during the photoshoots. I use
Visatec 800B flash units for lighting.

I've got a pretty good idea why you have the red cast. However, I
don't know why it might have changed recently.

Studio flash units tend to have a colour temperature of about 5500K,
the same as typical daylight. However, on-camera flash units
sometimes have a somewhat higher colour temperature. So, with studio
flash you often get more accurate colour balance by using the daylight
setting.

Andrew.