I also experience the same problem on a fresh Ubuntu 16.04.5 LTS install. Fresh meaning: installed 6 days ago, in the meantime had to do 'sudo apt install --reinstall ubuntu-desktop'. Since then, I'm having this issue every time I lock my screen.
I also experience the same problem on a fresh Ubuntu 16.04.5 LTS install. Fresh meaning: installed 6 days ago, in the meantime had to do 'sudo apt install --reinstall ubuntu-desktop'. Since then, I'm having this issue every time I lock my screen.