Using older versions of Outlook for Mac in business environments
I don't work for Microsoft and can't fix it. (If I did and could, there is a lot I'd change.) Yahoo/AOL made a change to their server that does not play nice with the legacy outlook. Microsoft needs time to figure out what happened, and fix it and make sure the fix doesn't add more problems.
Last year around this time they upgraded their servers a set a limit of 10,000 messages per folder among other things. They've been tweaking it fairly frequently ever since.