Damn kids. Get off my lawn!
let me preface this by stating that I am a network engineer with 25 years of experience - adoption is a common term used in my industry when joining a device to a network, this can be a switch, access point or even a firewall.
With this specific issue, the devices do appear to be bricked as they are not able to communicate with the sonos servers for whatever validation they are receiving.
Is there any status light activity? Network Port light activity? Have they requested an IP address?
it give the message that unit adopted however it may not show up…. unplug for 10 seconds and plug back in.
It then gives the green flashing light showing that it is ready to adopt again
It’s a bit of a long shot, you might already have tried it and I can’t give a sensible reason it might help as they sound like they have reset properly with the green led, but have you tried a factory reset with the ethernet plugged in.
I had a couple of devices that didn’t factory reset properly until they were attached to ethernet when resetting them.
Interesting that you use adopt that side of the pond. Over this side we have always used on-board or enrol for as long as I can remember. It did generate some silly comments when personnel/HR first started talking about on-boarding staff rather than just setting up equipment and training up new starters 
Have you reset or reinstalled the controller after the successful adoption? Sometimes this helps — along with a reboot.
I have uninstalled controller - even wiped ipad with nothing but the controller - reset the speaker plugged in.
I found the following site with info on how to access the speakers via browser however the affected speakers reject any connection wired or wirelessly - they do in fact connect to the network wirelessly, but cannot contact the sonos servers.
Hidden Sonos Interface
There is a lot of information in here
