Custom login page built with react wont let me save


I have a small react application I intend to use with the lock library in order to customize our login experience. I’ve built it into a single html page, without implementing the lock yet, and pasted it instead of the default html in place.
When I try to save the new content it gives me a generic " Error! Unexpected failure trying to save the changes, try again later." with no more details.

Are there any limitations to replacing the existing html on the custom login page? Is there some documentation I’m missing?