Microsoft apologises for Windows 10 for Phone Preview download problems and promises it won't happen again

Update: It seems Microsoft have fixed the download problem. All is working, although remember: this is a big and slow download. It will take at least an hour in all. *And make sure your battery charge is at least 40% or installation won't happen*
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Microsoft has issued an apology for server problems that have been stopping Windows Phone users from downloading the latest update for Windows 10 for Phone Preview..

It basically means the update only works for users who've already installed the first build of Windows 10 for Phone and who have selected Fast Track in the Windows Insider programme.

Redmond says it will take a couple of hours to fix:

UPDATE: We discovered this morning that there was an error in the database for provisioning record data, which is required for new phones to join the Fast Ring through the Windows Insider app. The team has been working to correct it but the load on the provisioning data servers has made all operations extremely slow. To fix the issue, we are taking the service offline and updating the provisioning record data for all 5100 phone POPs. It will take approximately 2 hours for us to make the update, and validate that everything is working. This is a different service from Windows Update, so Insiders who are currently on 9941 should be able to update their phones if they are already in the Fast ring. If they are in the Slow ring they will encounter the same issue with provisioning data. We apologize for the delay on this and are working to ensure it never happens again.

So....have a cup of tea....relax...and try again later.

(Source: Windows official blog)