Two flashing greenoptions:

Flashing green
The Sonos product is powered on and ready to be set up. It has not yet been configured or connected to a Sonos system.

If your Sonos product is not brand new, a flashing green light means that the product was reset and is ready to be set up.

Slowly flashing green
This light appears on home theater surrounds that have been disabled using the Sonos app.

You can re-enable your home theater surrounds in the Sonos app for iOS or Android in the 

 Settings menu. Under My System, choose your home theater product from the list, then select Surround Audio and re-enable the Surrounds option.

Thank you, Stanley, but this is not part of a theater system. Just two paired speakers and one went out with the update.  

Logic would say the stereo pair has been broken. Here are a few options in the order of best suggested. However the choice is yours:

1. Do not factory reset the speakers
2. If the speakers were set as a stereo pair and for some unknown reason they no longer function as such I suggest you run a diagnostic, make note (do not post in a reply) of the reference ID and call Sonos tech support
3. If speakers still show as a stereo pair in the app I suggest removing them as a  stereo pair. The speakers should revert back to separate rooms (sort of). The speaker flashing the green light may not so set it up as a room; as if you were adding a new product.  If that is successful then set them up as a stereo pair again

Thank you! 

Slightly different approach :

First, I suggest that you reboot both speakers. If this does not resolve the situation, check for updates again, If there is still an issue Factory Reset ONLY the problem unit, then add it to the existing system and rebuild the pair. Don’t setup a new system. I expect that the issue will be resolved at this point. If not, submit a diagnostic and contact SONOS support.

All fixed- thank you!