As no answer has been marked as correct yet; this could be this issue with running xscreensaver on startup
as @peterx14 says here, many of the tutorials for setting up xscreensaver provide incorrect information.
They tell you to set up a screensaver.desktop file with the following command:
sudo gedit /etc/xdg/autostart/screensaver.desktop
and then place inside it the following information:
However, for some reason, in many of these tutorials, the above command is incorrect. 'Application' has been misspelt as 'Applicaton'. This causes the startup command to not work.
Therefore, if you have already entered the command
into your startup list, it would not work as xscreensaver is not set up with the type application.
To find out if this is the issue: first run
locate screensaver.desktop. This should, somewhere in the list, tell you where screensaver.desktop is. On my computer it's in
/etc/xdg/autostart/screensaver.desktop, then open it in an editor (terminal 'nano' may be best as you will need sudo commands to open it) and change 'Applicaton' to 'Application'.
Also, different tutorials (such as liberiangeek and Radu's response) suggest using:
as the fix above uses. This depends which value you have within your screensaver.desktop file on
line 4: Exec=xscreensaver -nosplash.