Oh my, a login, wifi, device add sequence succeeded in claiming the device! After logging out and logging back into the app, I have a working Tinker interface. Just for the record, I reset the board, put it into setup mode, then used the commands
particle login
particle identify
particle serial wifi
particle device add ID
Looks as though the two dfu-util commands got the board back to a good state.
Thanks!
maf
Would you mind changing this topic to include [SOLVED], seeing as your board is up and running?
Also, big shoutout to @kennethlimcp for chiming in and helping out 
!
Agreed on the shoutout to @kennethlimcp - this diagnosis and fix should be in a short How-To page!
maf
Diagnosing it is hard, as you might have notices, due to different interpretation of the colors/sequences. Having a short how-to page is definitely something that should be worked on. There’s a guide in the troubleshooting section, but that is evidently not sufficient.
@christine, this topic might be a good example of what kinds of troubleshooting information is necessary in the docs 
This just fixed my expired key problem on a Photon! 
Will this method also work on the older Spark Core?
place the device in DFU mode
download https://dl.dropboxusercontent.com/u/36134145/particle/junk.der9
dfu-util -d 2b04:d006 -a 1 -s 34 -D junk.der
download https://s3.amazonaws.com/spark-website/cloud_public.der8
dfu-util -d 2b04:d006 -a 1 -s 2082 -D cloud_public.der
reset the Photon
I’ve having the same issue and tried your solution.
All goes well till the step "particle device add ID [mydeviceID]"
I then get the respons:
Claiming device ID
Failed to claim device, server said: [object Object]
Any idea how to solve this?
it’s not
particle device add ID [mydeviceID]
but
particle device add [mydeviceID]