I found this happens when you have your website at the root instead of a virtual directory.
I have found the error and submited it to Lopocon.
The function DotNetNuke.Common.ApplicationPath now returns nothing if the website is running at the root.

If you can re-compile the provider, in FCKGalleryBase.vb, starting at line 140 the code should be as follows:

If rf.StartsWith("/"c) Then
   rf = rf.TrimStart("/"c)
End If
'Have to get rid of application path. This fixes problem with default root directory
Dim strAppPath As String = DotNetNuke.Common.ApplicationPath
If strAppPath <> "" Then
  rf = rf.Replace(strAppPath, "")
End If
RootFolder = rf

And for people who do not know how or who can not recompile ????

DV