ConnectX-4 Lx reports Cable is unplugged with FS.com SFP28-25GLR-31 modules over 40 m OS2

A Mellanox ConnectX-4 Lx (MCX4121A-ACA_Ax, firmware 14.32.1900) in a Ryzen desktop (Fedora 41 and Windows 10) detected the interface but reported Cable is unplugged with two FS.com SFP28-25GLR-31 modules and a 40 m single-mode LC duplex patch; a MikroTik router recognised the same optics. The poster reseated everything, swapped cables, reset configuration with mlxconfig and mlxfwreset, re-flashed firmware with flint, disabled auto-negotiation, changed FEC and moved PCIe slots. mlxlink showed status opcode 36 with PHY FW group opcode Other issues.

• Hardware: Mellanox ConnectX-4 Lx MCX4121A-ACAT, MikroTik router
• Transceivers: FS.com SFP28-25GLR-31
• Form factor: SFP28

Symptoms / messages:

Cable is unplugged
mlxlink status opcode 36 / Other issues
no link