I agreed with Barry’s Solution. But, in case, if the above solution does not help to resolve your issue you can also try to reset the Navigation Pane in MS Outlook.
To reset the Navigation Pane: Press “Window” button + “R” >> Type “outlook.exe<space>/resetnavpane” without quotes in run dialog box and press enter.
When I was searching about your query on Google, I found an informative article on “Outlook Common Errors”. So, I want to share with you that article to get aware about most common Outlook error.