When trying to open a external link in evince (document viewer) I got this message:
Failed to execute child process "exo-open" (Permission denied)
It seems that AppArmor is preventing evince from executing exo-open, which would handle the link.
Open a terminal window and run (two lines):
sudo ln -s /etc/apparmor.d/usr.bin.evince /etc/apparmor.d/disable/usr.bin.evince && sudo service apparmor restart