After upgrading to 18.04, I cannot launch terminal or nautilus from the favorites side bar. Xterm, however, is fine, and from there I can execute at least nautilus. Running
gnome-terminal in the xterm window do nothing, except the warning
"AT-SPI: Could not obtain desktop path or name"
Ctrl + Alt + T won't start the terminal, either. Nothing happens.
PS: I went from Ubuntu 16 through Ubuntu 17.10 without really using 17, perhaps I had the problems there, too.