It happened to me and it turned out to be the (presumably unintended) result of an automatic Microsoft update of the Office suite. I went to System Restore and returned to an earlier version before the update and my Outlook problem disappeared.
Unfortunately, I then developed an Excel problem, where it kept directing me to the current version of Excel and wanted me to pay for it. It also started naming my Excel files with an .xlsx extension, rather than the prior .xls, which I suppose contributed to the problem. I resolved that issue by finding my PC’s list of apps, finding Office, and clicking to Modify, which started a program to fix issues, and it fixed mine.