\\server is not accessible. You might not have permission to use this network resource. Contact the administrator of this server to find out if you have access permissions. The request is not supported.
For various reasons we could not change the Samba server security mode to ADS. A workaround, however, was found here.
On Windows Server, start Local Security Policies, navigate to Local Policies --> Security Options. Open the "Network Security: LAN Manager Authentication Level" setting and set it to "Send LM and NTLM responses".After that, remote Samba share is accessible. No reboot necessary.