-are you rendering render elements? Yes and they were saved fine.
-are you using any objects coming from a different renderer (for example a Vray camera)? Yes, Vray Camera and HDRI, our scenes are always came from vray.
-are you using any 3rd party plugins (for example multitexture)? yes a lot, multitexture, multiscatter, railclone, forestpack...
-does this happen always, or only in specific scenes? always, if we render via backburner
-are you sure that the PCs are allowed to communicate and access all the required files? yes, renderings looks fine and nothing is missing...
Thanks a lot for your help!