Thank you Lorenzo for the description!
This has let the developers identify what is happening, and it sounds like a known issue. The issue happens if you open C4D, and start rendering to the PV without ever having opened the VFB - in that situation, the VFB will be black. The solution is to always open the VFB after C4D starts up - that can be in any scene, and you don't need to render, the VFB just needs to have been opened one time before rendering starts.
While nothing can resolve it (for now at least) once rendering starts, with that small addition to a daily workflow (as soon as C4D runs, open the VFB, don't need to render, can be any scene, and can close the VFB right away), you will be able to avoid running into this one in the future. Hope this helps, and ty for your reports!