Some Lumia 640 devices can't currently install the Windows 10 for phone preview
Microsoft has admitted that some Lumia 640 models won't be able to install the Windows 10 for phone preview, due to an issue with the Windows Phone 8.1 OS build on those units.
Microsoft's support forums have the information on this problem:
The post adds that Microsoft is "actively reviewing possible alternatives" to solve this situation. It sounds like this problem is similar to the one that was encountered by Lumia 520, 521, 525 and 526 devices a few days ago. That issue, which was related to a bug in the Windows Phone Recovery Tool, has since been resolved.
Source: Microsoft; Thanks to Luke for the tip!
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