This problem occured when I attempted to add my freshly deployed github pages site(accountname.github.io/repository) to the list of allowed callback URI's. I pasted the URL into the textbox and saved. When attempting to log in through the website, however, I got an error message and the error page:
I've tried multiple times to remove the URL from the list and add it back, I've also tried to change the callback and redirect URL's in the sign-in and sign-out button code. I was expecting it to complete the authentication process and show the log-in page, but to no avail.
Error solved itself, tried changing application settings from Single-Page to Regular Web