If you were one of the unlucky Lumia 640 or HTC One M8 owners that couldn't install the latest build of Windows 10 Mobile when it was released last week, Microsoft's Gabriel Aul says that the issue should now be fixed.
Aul made the announcement on Twitter earlier today, saying the following:
"#WindowsInsiders with Lumia 640 or HTC One running GDR2 - we now have an upgrade hop for you to build 10080 so you can install."
At least for the Lumia 640, the problem stemmed from an issue with the upgrade path not being recognized by the update server. However, it looks like Microsoft has worked out the kinks, and users should now be able to update from Windows Phone 8.1 to the Windows 10 build 10080.
Source: Gabriel Aul (Twitter)