Ran into this on the Release Preview of Windows 10 1704. From the Insider forums, user Win10Nuc2 provided this solution.
Use the manual update procedure from the command line for Defender – looks to either use a different source, or flush a cache before downloading new updates, or something of the sort.
1. Open an administrative PS session
cd 'C:\Program Files\Windows Defender\'
.\MpCmdRun.exe -signatureupdate -mmpc