This post is a mental brain dump of the issues I encountered when configuring Azure B2C in a new Power App Portal or Power Pages environment.

Error when trying to login via B2C:
.powerappsportals.com/signin-aad-b2c_1
Page not found
Open the Site Settings, filter the page to only display settings for the Website you are working with, then search for: authentication/openidconnect/

You will see all of the items related to the B2C items you set up on the front end. I’ve noticed, for whatever reason, that the settings don’t always sync with the backend.

To fix the Page not found error I switched Authentication/OpenIdConnect/AAD-B2C_1/RegistrationEnabled to true . Open the portal in a new private browser and try logging in again.

Power App Portal authentication settings not syncing.
If you create a new B2C user flow and want to update your portal settings, save yourself time and open your portal site settings, search for authentication/openidconnect/ and update the setting there. For some reason, updating portal settings in the settings page don’t always sync to the backend.

Failed to create/update site setting for default provider.
If you get this error, try setting a different provider as the default, then try your original selection again.

If you log in via B2C and see this, check the setting in the screenshot, and purge your cache.
Registration is disabled
Invalid sign-in attempt.