I recently download IPB 4.3. It installed perfectly and I was able to access the admin. I then viewed the front page and attempted to login. Immediately I received an ERROR 500 and then Access Denied on any refresh thereafter. Additionally, anyone else who trys to visit the website receives the same error.
I rebuilt the webapp (nginx and appache) solved the issue. Im not really sure why that had happened. I am guessing something with the .htaccess got stuck.
For anyone else that runs into this issue, my solution is pretty isolated to me. I have a server panel on a VPS that lets me rebuild isolated containers. Thats what I did but I have no solution for any other type of hosting.