Today I had a user that just had this problem pop up seemingly out of nowhere. I tried repairing Outlook, starting it in Safe Mode, disabling all Add-Ons, finding and deleting out Outlook’s temp files, and the problem still persisted.
After a brief web search, it appears the latest batch up Windows Updates, specifically KB3097877 that had installed automatically last night, was the culprit. Uninstall it, reboot and hopefully you’re good to go. Here are my references: