Ok I have done some testing and I have narrowed the cause down to the pppoe settings. After a reflash I can change any setting within the interface and it will work properly, until I choose pppoe (wired) and put in my username and password. After that the interface will always kick me out to a session expired page and ask for my password.
Edit: The interface works fine if I put in a fake username and password. Weird behaviour only seems to begin after a pppoe connection has been made.