Microsoft today released Windows 10 Build 10581 to Windows Insiders. This build has some known issues. If you restore a backup from a phone that had a different resolution, you may end up with a corrupted Start experience. If your phone looks something like the above, then follow the instructions below to resolve:
The workaround is to swipe down from the top to bring up Action Center (which won’t be corrupted) then Settings > Personalization > Start and apply a different background picture.