This is my page: http://cityhallreview.com/join/
I uninstalled Newsletter and deleted the 4 database files. Then reinstalled Newsletter.
I immediately went to Configuration and clicked on Advance Settings. Without changing any settings, when I click on Save it sends me to 404 error. The same thing happens if I click on Configuration while on the Advance Settings tab.
I am then locked out of WordPress dashboard. I have to delete my website cookie and log back in.
Hope you can figure this out.