Purple renders on a render farm

Hi there, i have a problem with one of my current projects.
It’s a a 300 frames animation, made with Blender 2.78a & Cycles, output as PNG, rendered with Sheep It renderfarm.
It needs no external data, it’s just a particle system duplicating an icosphere with a material that plays with particle inputs, refraction shader node and emission shader node. Nothing really complicated except the amount of particles. As far as i know, i didn’t used addons, experimental features or “things that certain configs can’t support”.
My project started to render really quickly as soon as i uploaded it in sheep it, under the name “Particles_01” (what a fancy name).

If i watch the rendered frames, i can see that some of them are partially purple :


I thought that kind of “purple errors” occurs when a texture file is missing or when the computer’s PU doesn’t supports well something. But as i said above, i doesn’t use textures nor experimental features. It’s just icospheres, particles (duplicating the icosphere) and a material with emission & refraction.
And if i render these purple frames by myself, they render properly, not in purple.

I do not understand why this problem is there and how avoid it (except rendering all by myself or paying a render farm).
I know the problem can be from sheep it itself, but since there is way more people here, i have more chances to have some answers.

Thanks for your help. And sorry for my pickle English.