I got the same as Joel.
What worked for me was to run the Windows Install(er) Clean Up Utility
(available from MSFT) and remove the entry for Windows Defender Signatures.
Then, updating from WU or AU will fail, but updating from within Windows
Defender will succeed.
You should be able to find the signature update listed in that, and remove
it. Then try the update from with Windows Defender--help, about, check for
updates.