Users of WSUS servers who try to run updates on clients using Windows 8 and 8.1 will face the 800B0001 error code. This code tells you that due to the fact that Windows 8 uses a newer WUClient than WSUS 3.0 SP2 Server can offer, the client doesn’t trust the server, resulting in the aforementioned error code.
A lot of users were frustrated with this issue, and initially, the only solution was to wait until Microsoft started distributing a new WUClient that will fix the compatibility issues. However, after a while, Microsoft released an update for Windows Server Update Services, which was specifically meant to fix this issue, along with a few other quirks and bugs.
At this point, the only (confirmed) solution is to download the KB2734608 update for WSUS, and install it on your WSUS server. This will allow clients running Windows 8 and 8.1 to be able to use Windows Update without any problems.
Install the KB2734608 update
This error is one of those situations where Microsoft acknowledges their mistake, and comes out with a solution in a relatively quick amount of time. It is recommended that you synchronize all WSUS servers after applying this update. If you have a hierarchy of servers, apply the update and start synchronizing from the top of the hierarchy on down. You can download the update here. Another thing you should note is that the WSUS must be in a good working state for the update to work, and must be able to synchronize from Microsoft Update. The clients must be able to communicate with the WSUS server as well.
The file you are downloading from Microsoft’s website listed above is an executable file. This means that installing it is as simple as double-clicking it in your Downloads folder. Wait for the wizard to complete, and reboot the WSUS server. You can now start using Windows Update on devices running Windows 8 and 8.1, as this update also upgrades the Windows Update Agent, and fixes the error 800B0001.