Problem with Mount Drive on Windows Server 2012 R2

Hi,

Recently migrated an old Windows Server 2003 R2 DC/File to a new Windows Server 2012 R2 Foundation.

Now we have an unusual problem with mount drives on Users computers.

All Users have a mount drive with Letter Z:, the Public Folder on Server shares. As an example, one User has opened one file from the shared folder on Server, let say Z:\User_folders\Folder\File.XLS

When user is going to save the file after edit, Microsoft Excel or Word opens Windows Explorer and waits user prompt where to save the file.

Also some users have shortcuts to folders of the mount drive at their Desktop and sometimes when they try to open the folder, windows prompts the following error.

Restoring Network Connections

In that moment, user can ping the server and nslookup is working.

I've tested both mount the Share Folder with NETBIOS hostname and virtual IP Address.

From the Server side is Windows Server 2012 R2 Foundation Full Updated and from the clients side we have different versions of Windows-Office (Windows XP, Windows 8.1 - Office 2003 Professional Plus, Office 2007 Professional Plus).

Nice to say that before Server Migration we haven't this problem at all.