It's because you are using SMBv2 instead of SMBv1, as has been discussed at length previously in these forums.
If this problem is happening on Windows 10 PC's then you have to enable "SMB 1.0/CIFS File Sharing Support" in "Turn Windows features on or off".
https://social.technet.microsoft.co...ble-smb1-in-windows1o?forum=win10itprogeneral
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