The problem that I am experiencing (and many others like me as I have noticed in several fora) is that the "source" when selecting an image screensaver blocks at "loading" while nothing happens and no (folder or aperture library) source can be selected. The problem started basically since Apple blocked the possibility to use a folder with images in subfolders as source for a screensaver as used to work perfectly in Snowleopard. But while Mountain Lion only works with iPhoto or Aperture libraries it made the system susceptible to windows like errors.
When selecting a slideshow-screensaver, the source remains as "loading" where no options are given to select anything. It used to work until a few months ago, but I have no clue why it stopped or how I can get it to work again. When the slideshow screensaver is selected I get an error message telling me there was an error connecting to a netwerk server. No clue why it actually needs that server (which died some time ago). If i shutdown systempreferences while the slideshow is still selected, each time it tries to start the screensaver I get the same warning that there is a problem connecting to a (dead) airport extreme.
I have tried to reinstall Mountain Lion, rebuild a new Aperture image library, remove certain .plist files that may be connected to this as was suggested in several fora, nothing works. I cannot imagine that Apple forces me to abandon Mountain Lion and to go back to Snowleopard over a screensaver.
A hint to third party developers: please create a simple slideshow screensaver where you can select a folder including subfolders. It can't be that hard to create (although Apple fails completely).
Move your photo library aside and create a new, simple one as you did in the Guest account. Test.