***EDIT***
4/21/2015 @ 2:30pm PST
All,
With the changes that we made to the WPRT and the ensuing successful updates we've seen since making those changes, we have now re-enabled the update for the 520/525/526 devices.
Users waiting to upgrade to Windows 10 Technical Preview for Phones may now take the upgrade as desired. If you are already on the Windows 10 Technical Preview and registered to the Insider Fast group, you will now find a new build available for your device.
Learn more about this new build here:
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Old Thread title: (Update availability paused for Lumia 520/525/526 users)
We are seeing some reports of failures on Lumia 520/525/526 devices when trying to roll back to Windows Phone 8.1 using the Windows Phone Recovery Tool. We are pausing build availability for these devices temporarily while we investigate the issue and work with customers who have reported problems.