Error:"Exception: System.IO.DirectoryNotFoundException" when Toad for Data Analysis is started. (4273257)

When Toad for Data Analysis is started, the following error message appears:

"Exception: System.IO.DirectoryNotFoundException
Message: Could not find a part of the path 'X:\Windows NT 5.1 Workstation Profile\My Documents\Toad Persistent Cache'. 

Stack Trace:
at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
at System.IO.Directory.InternalCreateDirectory(String fullPath, String path, DirectorySecurity dirSecurity)
at System.IO.Directory.CreateDirectory(String path, DirectorySecurity directorySecurity)
at Quest.PersistentCache.DataEngine.set_CacheDirectory(String value)
at Quest.PersistentCache.PersistentCache.Connect(String privateDirectory, String shareDirectory, Boolean excludeHiddenColumns)
at Quest.Toad.DatabaseBrowser.ObjectPaletteControl..ctor()
at Quest.Toad.Analyst.AnalystBrowserControl.InitializeComponent()
at Quest.Toad.Analyst.AnalystBrowserControl..ctor()"