Please read my post here and check if, the situation is solved when providing ffmpeg as described in the post. 
If it does not solve the issue, could you provide the log files (ambulance icon in the Roon Server web panel)?
Please read my post here and check if, the situation is solved when providing ffmpeg as described in the post.
If it does not solve the issue, could you provide the log files (ambulance icon in the Roon Server web panel)?