Lumia 520 temporarily not allowed to get Windows 10 Technical Preview for Phones
UPDATE 22/04/2015 8:50 AM GMT: Microsoft has released details about how to recover bricked Lumia 520 phones or phones displaying a red nokia logo. Details here
UPDATE 21/04/2015: Microsoft has solved the bug and it is expected that tonight (GMT) an update will be available and will allow Windows 10 Technical Preview to run on Lumia 520, 525 an 526!
Last week Microsoft made Windows 10 Technical Preview for phones build 10051 unavailable for Lumia 520, Lumia 525 and Lumia 526 due to a problem that made the phone unusable after reverting back to Windows Phone 8.1
The users reported that their phones were bricked after trying to use the recovery tool to go back. Microsoft is aware of this and they are working on a fix for that and some other well known problems.
In the meantime, although the phones still appear listed on the list of supported Windows 10 Technical Preview phones, it has been reported that the users despite being on the “windows insider fast” ring they are unable to get any updates.
At this stage it is advisable to wait until the first update is available in “insider slow” before trying to install WP 10 Technical Preview in devices with low memory.