I was setting the gtk greeter and decided to remove the session indicator. And after that, the problems started again. I removed the ubuntu-desktop package, leaving only Mate Desktop but the "error on logon" message continued. I decided to return the session indicator and the problem disappeared.
The indicator provides no role for me and my users because only MATE is installed now, and I set this in /etc/skel for default. But serves to mark a desktop to start, option that has no effect on the working environment but avoid the session error.
Today, lightdm-gtk-greeter fail too.
I was setting the gtk greeter and decided to remove the session indicator. And after that, the problems started again. I removed the ubuntu-desktop package, leaving only Mate Desktop but the "error on logon" message continued. I decided to return the session indicator and the problem disappeared.
The indicator provides no role for me and my users because only MATE is installed now, and I set this in /etc/skel for default. But serves to mark a desktop to start, option that has no effect on the working environment but avoid the session error.
Sorry for my english