Sonos is merely attempting to send the data entered to a SoundCloud authorization server, and displays what is returned to them. My guess would be either your Sonos speaker is being blocked when it attempts to reach the SoundCloud auth server, or the code is indeed incorrect. 
 

You may want to reach out to SoundCloud, or submit a system diagnostic within 10 minutes of experiencing this problem in the Sonos controller, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.

My brother has the main account for our whole families Sonos. We are with 6 in total so we can't log in with multiple accounts. Spotify works from each personal app, and we want the same with SoundCloud.

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So, what did Sonos say when you called them? Sonos doesn’t have much, if any, control over how Soundlink sets up their account system. Sonos is an aggregator, they publish an API that other companies implement to be part of the whole Sonos ecosystem. It might be worth looking at the Sonos partners page. Ultimately, though, it’s up to SoundCloud to make the account linking choices, and not Sonos.