https://www.bleepingcomputer.com/news/microsoft/microsoft-releases-may-office-updates-with-fixes-for-auth-issues/

Microsoft released the May 2020 non-security Microsoft Office updates with fixes for several issues and performance improvements to Windows Installer (MSI) editions of Office 2016 and Office 2013.

For instance, the KB4484328 update fixes an issue leading to blank authentication prompts being displayed when offline auth is enabled in Microsoft Office 2016