Here is what Microsoft has shared in the forums.
Thank you again for sharing your recovery experience with us and reporting the QHSUSB_DLOAD bootloader errors. Thanks to you, we have good news to report.
Our investigation has helped us understand the conditions in which the error could occur. We found two potential causes for this state with the most common being addressable with a software patch which we’ve released today.
With today’s update, we’ve added support for recovering most devices already in this state. It’s important to note that this update applies to Lumia devices only and will not address phones that may be experiencing hardware related issues.
Thank you for being an Insider!