This problem is encountered by many users of Outlook 2013 and it is caused by an update released on September 2013. This update is not safe and contains bugs. In order to fix the problem, navigate to Start > Control Panel > Uninstall a program > View installed updates, select KB2817630 from the list and then click Uninstall. If you can't find the KB2817630 update, navigate to Microsoft Download Center and download and install the update released on January 2014.