Outlook on desktop Office 365 "not responding" after minor change in viewing reader pane

I made a minor change in the viewing reader pane - made e-mail to be marked read when moving to next e-mail in list. Now get slow response/no response to a simple action such as moving from the Inbox to the Sent box.

I have closed and re-opened Outlook with no change. I have re-booted with no change. I downloaded and ran the Diagnostic Tool and the tool found no issues. What is my next step?