Check on your server and on your clients these settings:
HKLM\system\currentcontrolset\services\lanmanserver\parameters
HKLM\system\currentcontrolset\services\lanmanworkstation\parameters
Requiresecuitysignature
Enablesecuritysignature
If the server requires the signature, enable it on WS.
If you are not sure, post the server's values and WS's values.