App hosting + custom domain: /__/auth/ paths give 404

[Jack Cogdill]

Hi,

I set up app hosting with a custom domain (subdomain). Everything is working in terms of loading the web app and SSL works too. However, when I click sign in, it results in 404. I am using the GoogleAuthProvider. The app is built with nextjs 14. It works on localhost in development.

For signInWithRedirect, I get 404 for the request to /__/auth/handler ...
And for signInWithPopup, I get 404 for the request to /__/auth/iframe ...

I believe I have followed all the instructions for setting up a custom domain, including adding my custom domain to the Authorized JavaScript origins and Authorized redirect URIs for the OAuth 2.0 Client ID, authorized domain in the Authentication settings, and in the app hosting config. I have tested in Chrome, Safari, and iOS Safari (and incognito for all). None of them work.

Is there something I'm missing?

Thanks in advance for the help.