Found a solution on Reddit that worked for me:
Make sure it’s not currently scanning (let it finish to where you get the error) then navigate to the World of Warcraft\Data\data folder (ex. C:\Program Files (x86)\World of Warcraft\Data\data), sort by file size, then delete any 1 KB file, then start the update again. For example, I had a file named “data.100” that I deleted. The file came back after it successfully updated but everything works as it should now.