Converted files cannot be written to the disk

Hi. I run PMS 1.0.3.273 on a Nvidia Shield TV and everything runs fine except I can not make optimized versions of any of my movies.
The movies are located on a shared network drive (USB-drives on a Raspberry PI) and I get "Converted files cannot be written to the disk". The shared folder has rights drwxrwxrwx so it should be writable? I can write to the disk when accessing it from a Windows PC. Any ideas?