It does look as though the 2413U powerLinc is malfunctioning. I tried to get the green light to come on for an hour (attempting a hard reset many times). Then i just left it plugged in...1 hour later the green light lit up, seemingly out of the blue.
I also was able to hotplug other usb devices into any of the 4 mac mini ports with good results, as long as the device wasn’t the PowerLinc.
New Insteon 2448A7 dongle arrives tomorrow.