Try enabling signing on the storage system for SMB 2, and let us know.
Data ONTAP 7-Mode: filer> options cifs.smb2.signing.required on
This could be the cause : When the client sends a request to the storage system, the Secure Negotiate feature requires that any error response returned be signed correctly. If the storage system returns an error that is unsigned (smb2.flags.signature == 0), the connection will fail. This will occur even if the storage system does not have SMB 3 enabled (or does not support SMB 3). Clients using the 'Secure Negotiate' feature will expect all error responses to be signed even if those clients only allow SMB 2.0 and SMB 2.1.