When using a Guest session, after the screensaver kicks in I am permanently locked out of the existing session and I lose ALL data I have been working with.
I use the Guest session and do work, I leave the computer for a moment, I return and the screensaver is on and I am asked for the Guest sessions password. But there is no password for a Guest session so I am permanently locked out of the session and lose all the data I have worked with.
Hi,
try running "dpkg" per the update guide (a network cable connection is required!):
That might fix the problem for you, make sure your software sources download location is set to "Main server"!. 
Can confirm this happens with 14.04.2 LTS, 15.04 and also 15.10 (Beta 2).
I thought I saw something about this bug in the past, but I can’t find anything, so I reported this to Launchpad:
https://bugs.launchpad.net/ubuntu-mate/+bug/1502676
(To help raise the bug’s heat, click on “This bug affects me too”)
Thanks. I guess no one uses guest sessions since this has gone unnoticed.
Your right, I just use the guest session for troubleshooting. So I just tried it and can also confirm this behaviour in 15.10.
I hope this kiosk setup tweeted the other day considered disabling the screen saver too:
https://twitter.com/ubuntu_mate/status/648862777206022144
As suggested by this bug report, there is a workaround for 14.04, 14.10 and 15.04.
Add this command to /usr/lib/lightdm/guest-session-auto.sh
gsettings set org.mate.screensaver lock-enabled false
I can confirm that the screen saver will no longer locks the screen but this does not prevent the session from being "locked out" if someone tries to purposefully lock it via the menu or by pressing CTRL+ALT+L.