I do have an extra PLM, but I won't be able to unplug the current one until tonight. However, it is still mostly working. I am able to control devices via the web interface and bring up the admin console, as long as I don't re-boot it. If I do, then I'll have to power it down for a couple of hours before I can use it again.
As soon as I do anything that requires a re-boot (switch back to using DHCP, firmware update, etc.) it thinks the cable is unplugged and I have to power it off for a while.
I think this is consistent with a problem I've had with this box for a while. For the past few firmware updates, I've had to power cycle it to get it working. It would do the firmware update, reboot, and then it would not be accessible until I power cycled it. It's just getting worse now.
A quick update to note that it wasn't the upgrade itself that failed. It seems there may be a heat related issue with the network hardware itself. After sitting unplugged all night, I plugged it back in on a whim and it worked ... for a little while.