I’m having the same issue.  I’ve tried everything:

• factory resets
• ethernet cable
• creating a new sonos account with a different email address

Still unable to update eventhough both the beam and my sub-mini are showing white lights and connected to network.

Very frustrating

Have you tried rebooting your router?

Yes, rebooted router, rebooted beam, rebooted sub-mini, rebooted cable modem, rebooted mobile device

FYI - after an entire weekend of trying to get this working and countless factory resets I was eventually able to get it connected and running.   Here are the steps I performed.

1. Download and install windows Sonos app
2. Connect to Beam system 
3. Perform update successfully 
   1. This got me past the update nag screen on the mobile app
   2. Still had issues recognizing and registering the beam and sub mini fully
4. Perform factory resets on both devices
5. Connect Beam via mobile app (success)
6. Connect Sub Mini via mobile app (Success)

At this point I was able to register and set the appropriate room and connect the Beam to the Sub Mini.   It took a while to complete the above steps and everything was far from smooth however the key to the entire operation was using the windows app to update the beam which was impossible via the mobile android and mobile apple apps.

Hope this helps

Also, using an ethernet cable during the setup process may have helped as well.

This works for me… thanks!!!