Already looked at other possibilities. That change (from 0 to a specific number of Mb) definitely made a difference in my system.
I Have a laptop where I don’t experience the issue with the same file and even less RAM and also tried with the setting at 0 without issues.
I’m still puzzled about the whole thing … At least it works now and I can finish my project.
One thing that it seems to be definitely a factor (in the problematic system) is the use of vray lights. It all started when I added rectangular lights. I never had a problem with vray sun and HDRI dome lights. But I cannot figure out how the lights generate the problem, I can only say that they do.