This error also pops up when trying to Auto update your O2 UK device firmware using Desktop Manager 4.7. According to O2 UK, this is because their firmwares are on their own site and not RIMs. Have had no issues using this new feature with Vodafone UK devices (whose updates are on RIMs site).
Historically you had to download the necessary firmware update from your Mobile Network homepage or BlackBerry - BlackBerry Desktop and Device Software Download Sites
, then install it and run Desktop Manager.
With the new auto update feature of DM 4.7 this is no longer necessary. It will automatically pop up a message if a new firmware is available for your device each time you plug your BlackBerry to the computer.
Gives you the option to Update, then the same options as before to select which apps you want to install, downloads the appropriate modules for it and automatically installs it on the device.
In all takes approx 10-20minutes to download and another 10-20minutes to update the device.