Hair turns transparent/invisible while rendering

I have an issue where hair turns invisible during renders. I have no idea what causes it because it seems to be completely random (it just started happening again after having gone away for a few months).


problem render.png
931 x 910 - 735K
Comments
It happens to me too when I use "Interactive Iray", have you tried to use "Photoreal"?
You can find it in the "Render Settings" tab!
I use Photoreal by default. It might just be an issue with my computer, because rebooting the whole thing tends to work.