UDM Pro to US-16-150W over a 1 m Ubiquiti DAC: link light on the switch only, even with both ports forced to 1G

A UDM Pro (port 11 set to 1G) and a US-16-150W (port 17) connected by a 1 m Ubiquiti DAC (UDC-1) did not talk: the switch showed a link light, the UDM Pro showed nothing. Setting both sides to 1G and rebooting both devices did not help; reversing the cable and removing the parallel copper uplink changed nothing. Ubiquiti staff asked for ethtool eth10 and ethtool -m eth10 output. Another user had a UDM Pro freeze completely when a 10Gtek DAC to a 1G Cisco SFP port was plugged in.

• Hardware: Ubiquiti UDM Pro, Ubiquiti US-16-150W, Cisco SG switch
• Transceivers: Ubiquiti UDC-1 DAC, 10Gtek SFP+ DAC 0.5 m
• Form factor: SFP, SFP+, DAC

Symptoms / messages:

link light on switch only
UDM Pro freeze on DAC insertion