Three thoughts.
1. Have you tired to manually use the green "+" add button and then once the files appear use the green "ADD" button? If not, try that method.
2. With a blank disk in the tray, give us the results of:
Code:
~$ dvd+rw-mediainfo /dev/sr0
3. It could be a repository problem:
https://askubuntu.com/questions/7529...n-ubuntu-16-04