“– C4D uses linked files a lot, rather than storing a copy in your project. Is there any chance any of the linked files could have been changed by you or someone else (think shared drives, shared cloud storage etc) which could have had a knock on affect?
– Quite out there and unlikely but C4D has a daylight node that can match the light position and output to your current time and time zone, so rendering at different times of the day will produce different results!”
I think that one of these two points that Tom mentioned may be the reason, especially the first one if it is related to changing the location of some HDRI files used in the scene lighting.