I have now moved this cable from the N7K-F248XP-25E to an expander N2K-C2232TM-10GE and the issue does not appear.

Thanks for your reply. 1 - The host is actually powered off - from what I can see, every 30 seconds or so the link tries to initialize, comes up at 100Mbps for about 8 seconds, which is unsupported on the N7K-F248XP-25E.So the error makes sense. Then...