(formerly Insider Fast)

Version Release date Highlights
Version 1709 (Build 8518.1000) August 28, 2017 Known issue
Some Insiders report that they can no longer save files in Word, Excel, and PowerPoint. When saving, they may see a "Product Activation Failed" message in the title bar of the document, or the app may crash.

Source: Update history for Office Insider for Windows desktop - Office Support