CSS326-24G-2S+ and Intel 82599 NIC fail to link over an S+DA0001 DAC; switch-to-switch DAC only reaches 1G

With a CSS326-24G-2S+ (SwOS 2.6) and a server with an Intel 82599 card, two different S+DA0001 DACs and two Intel cards never linked; two CSS326 switches connected by DAC only achieved 1 Gbps. A later reader on SwOS 2.12 with an 82599 reported the same. No solution was posted. Hardware involved: MikroTik CSS326-24G-2S+, Intel 82599 (X520), MikroTik S+DA0001 DAC.

• Hardware: MikroTik CSS326-24G-2S+, Intel 82599 (X520)
• Transceivers: MikroTik S+DA0001 DAC
• Form factor: SFP+, DAC

Symptoms / messages:

no link between CSS326 and Intel 82599
switch-to-switch DAC at 1G only