I had the same problem using a Cisco vpn 3002 SSL VPN connection. I found this, It seemed to fix my problem. I ran this on a non production server. I'd use caution on a production machine since this will set all the DNN folders to all user access. Otherwise modify the script.

You can set folder permissions to everyone  for all folders in your filesystem through an sql script.
Run this script from host > sql, and all your folders, in all your portals will have read permissions for all users set.

**********************************************************************************************************************************************

declare @PermissionID int
select @PermissionID = PermissionID from {objectQualifier}Permission where PermissionCode = 'SYSTEM_FOLDER' and PermissionKey='READ'

declare @FolderID int
DECLARE FoldersCursor CURSOR FOR
  Select FolderID from {objectQualifier}Folders

OPEN FoldersCursor

FETCH NEXT FROM FoldersCursor
  into @FolderID

WHILE @@FETCH_STATUS = 0
BEGIN

   if not exists (select * from {objectQualifier}FolderPermission where (PermissionID = @PermissionID) and (FolderID = @FolderID) and (RoleID=-1))
   begin
   insert into {objectQualifier}FolderPermission (FolderID, PermissionID, RoleID, AllowAccess) values (@FolderID, @PermissionID, -1, 1)
   end
   FETCH NEXT FROM FoldersCursor
     into @FolderID
END