I "solved" the problem by registering for an Office 365 account, which finally -for now- allowed me to redeem the product code (for Office for Mac 2011), and then Now Word, Excel and PowerPoint will allow me - for now - to save changes and create new files. The most helpful Help page was:
Activate Office for Mac 2011 - Office Support
with which I 'signed in to an existing Office 365 subscription'.