Outlook for Mac Export Broken, What is the alternative?

First, try rebuilding your Outlook mac email database. Here is a link that will help you do so: https://support.microsoft.com/en-us/help/2360509/rebuild-the-outlook-for-mac-2011-database-to-resolve-problems

 

There are some third party apps also available that perform the backup function. One that came in top charts is this one: http://www.mailbackupx.com/how-to-scenerios/how-to-backup-outlook-mac-mails-on-mac.html