- Open Sitecore.config and increase the
DefaultSQLTimeout
setting to 2 hours (just in case). - Go to the Control Panel and run the "Clean up Databases" application for the Master and Web databases. Don't close the dialog until the cleanup is done.
The issue should be fixed after that.
I am fairly confident it is the "RebuildDescendants" part of the cleanup"RebuildDescendants" part of the cleanup that fixes the issue by rebuilding child-parent relationships between items.
A similar problem is described here: