I am trying out the Azure AD-B2C. The user signup/sign in is fine when the MFA is turned off. But when I turn it on, and the user tries to sign in and provides the phone number, and requests a text message by clicking "send code", I get the Http 400 error: size of request headers is too long. Anybody else have this issue?
The error HTTP 400: Size of header request is too long generally happens because there's too many cookies.
Azure AD B2C's login goes through login.microsoftonline.com, as does almost every Microsoft service (O365, Azure, etc). So if you've got several accounts that you've signed in to across these services, you're accumulating cookies that will cause this problem.
Clearing the cookies should resolve this problem. If this is happening on a recurring basis, you should edit your question to include details about the request and cookies in order to best figure out what's bloating the request and how to reduce it.