Thank you for your reply.
I’ll explain more information about the screenshot error.
More Information for the screenshot
Password login via OIDC-conformant clients with externally-hosted login pages is unsupported. Alternatively, login could have been initiated from the wrong place (e.g., a bookmark).
According to auth0 dashboard log, there was the message.
However, my customer didn’t use the bookmark.
Some leads of my mind
About my program, I expose login route, where my code kick the redirect method to the auth0 login form, according to the following comment.
And Then I used Classic Universal Login.
About the emergent solution
Many times I try, I solved the problem by using New Universal Login instead of Classic.(I changed it at auth0 dashboard.)
After that, our customer succeeds to log in, and doesn’t encounter the error (POST https://<subdomain>.us.auth0.com/usernamepassword/challenge 404 not found)
I wonder why it is…
Because I wanted to use New Universal Login, I think it is nice.
But as I will introduce SSO function in the near future, I want to use home realm discovery which New Universal Login doesn’t support.
So, I want to know why the error occurred for the case that I will have to use Classic again.