OSPRay Background changes in 5.7-RC1 when connected in client/server mode
In 5.6, OSPRay treats the provided background as an HDRI environmental map (warps the 2D image into a 3D sphere to gather lighting information). In 5.7-RC1 (when connected in client/server mode) this changes to keeping the background as a back plate (just a static image from which no lighting information is taken).
Ideally, you really need both (and apparently OSPRay 1.8 supports this) to be able to create a high-quality render. The request is to have an HDRI load to gather lighting from, and set a backplate load to set the static background.