These instructions apply if you're using an iPhoto library as the source of images for the screensaver.
You have one or more links to a disused server in the iPhoto library. Since the server is no longer accessible, those links are now invalid. You should be able to find them by a tedious process of elimination. First, back up all data.
Select about half of your photos and start to drag them. Don't actually move them anywhere; just drag them a little and release. If you get the warning, then some of the photos you selected are invalid links. Narrow down the selection. If you don't get the warning, select the other half of the library.
When you've isolated the invalid items, delete them and then select
iPhoto ▹ Empty iPhoto Trash
from the menu bar. Finally, rebuild the iPhoto library.
(Credit for this idea to Winston Moy of winstonmoy.com.)
In some cases, the above procedure may not work. Then you'll have to create a new library with the contents of the existing one. The easiest way to do that is with a third-party application called "iPhoto Library Manager," which you can find in a web search. From the application's menu bar (not the iPhoto menu bar), select
Library ▹ Rebuild
Books, calendars, and slideshows won't be preserved.
