The curious case of shit backdrop

On the off chance that someone is actually reading this, maybe you could help me figure out an issue I encountered in Iray today. Some three years ago, back in my Lux days, I created a simple backdrop in Blender which I’ve been using without a problem first in Lux and then in Iray. But today I’ve noticed that the transition area between the top and bottom part of the backdrop suddenly started to look like crap in renders:

I’m not crazy This transition used to be smooth. I think it’s possible the last DAZ Studio update is to blame (I haven’t used this prop in a while) but I don’t know, maybe there is some other explanation. Have you guys got any ideas?