NTLM IS THE PROBLEM!!!
The problem is that NTLM (or NTLMv1 called these days) is old and insecure. That's why everyone tries to ban this authentication method from networks and as such default settings changed over the years.
Windows 2008R2 should already default to NTLMv2 as does the Samba version OMV4 uses. So if such an authentication mismatch occurs one side has to be 'fixed'. Microsoft explains here how to do this on affected Windows versions: https://support.microsoft.com/…lm-network-authentication
So how did you solve this mismatch problem now? On the Windows or Samba side?