I seem to recall (from my brief DHO1000 ownership) that there are two firmware update paths -- one to download firmware from a cloud server, and one via a USB stick. And the way the two options were placed and named in the menus invited you to "take a wrong turn", asking for a cloud update when you wanted one via USB.
Maybe a DHO800 owner could clarify the right steps, and/or @leafi could describe in detail what was tried and failed?
EDIT: Found it in the manual. For a USB-based upgrade, you need to go via the Storage menu > Upgrade. Do not go via the "R" gearwheel icon > Upgrade icon, that will attempt a cloud update!