Hello Office and Windows Insiders.

Recently, Windows Insider builds 17060 through 17077 were released to a subset of Windows Insiders. These builds contain a bug which prevents Word files from being saved if any add-ins are running. Anybody running a Windows Insider build in this range that run Word add-ins will be impacted by this bug. This impacts all versions of Word, including production builds, as well as both traditional save and auto-save.

The issue has been identified and a fix is being worked on which should be released later this week when 17083 is made available. We highly suggest that anybody running one of the impacted builds update to 17083 as soon as it is released.

In the meantime, users impacted by this issue can use the web version of Word as a workaround.

-Jesse

Source: Known Issue Windows Insider Builds 17060 through 17077 - Word - Microsoft Community