SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON

[lefosg]

I am running the app on the completed-demo branch on Windows 10, and it doesn't seem to actually work. I use my mobile phone for registration and the error i get is: SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON. Sometimes this doesn't happen, but I get an error about attestation (#12 specifically, and I cannot fix that either even if I try the solutions provided). The other demos in webauthn.io don't seem to work for me as well.