Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Last weekend I finally got around to upgrading my system to Win 8.1 (via a clean install). That, of course, required reinstalling all my apps, including Office 365. Since I've done this, I have not had this inbox problem, at all.
This was a pretty drastic way to address this problem, but I had to do for other reasons. Luckily, it also fixed this problem.
Stan