No onboard on standby output

I use an AMD64 PC with the last update of Ubuntu 16.04 64 bits.
My keyboard is out and I’m using Onboard.
The Onboard is not displayed on standby output. I need to choose “change user” but some times after the login screen I have the standby output screen again and I need to restart the PC.
recently, with the installation of “master pdf”, the keyboard was inoperative to enter the password.
I have set the keyboard to load at system startup.
where does this error come from ?