Okay the suckless-tools package contains 'slock' which takes priority over i3lock in the in the screenlock script (/usr/bin/lxlock), so....
Uninstall suckless-tools and the screensaver should work again:
sudo apt-get remove --purge suckless-tools
if for some reason you cannot uninstall suckless-tools (maybe because something else you have installed depends on it),
instead run:
gksudo xed /usr/bin/lxlock
and comment out these 2 lines
elif which slock >/dev/null; then
slock &
so they now read
# elif which slock >/dev/null; then
# slock &
SAVE the file .. and now test the screenlock