The same thing happened to me. I am running 64 bit Windows 10, version 1709 (the Creators Update) and Office 2013. I've tried everything, running repair/reinstall on Office, removing and reinstalling Avast,and deep scans for viruses and and malware. This all started with the "Creators Update" on 12/21/2017 and 3 subsequent updates on 12/23/2017. Check your update history.
Interestingly, my wife's laptop is running 32 bit Windows 10 and got the same updates. But she is using Office 2007 and is having no problems.
Both computers are running Avast Internet Security version 17.9.2322 build 17.9.3761.0