Custom sign-in page for Azure AD B2C doesn't l

Question

We customised our Azure AD B2C tenant's Combined Sign-Up/Sign-In Policy to serve up our own login page. This worked across all the major browsers when we tested last week, but it stopped working today for some of our users on Chrome and Firefox.

We are getting this 404 error when some of our users browse to our home page and they get redirected to the login page (our B2C tenant and custom login URL is redacted but all other query parameters are unchanged):

https://login.microsoftonline.com/redacted.onmicrosoft.com/B2C_1_sign_up_in/api/CombinedSigninAndSignup/error?code=UX004&diags=%7B%22version%22%3A%222.0.0%22%2C%22user-agent%22%3A%22Mozilla%2F5.0%20(Windows%20NT%2010.0%3B%20Win64%3B%20x64%3B%20rv%3A54.0)%20Gecko%2F20100101%20Firefox%2F54.0%22%2C%22online%22%3Atrue%2C%22trace%22%3A%5B%22%231%20T005%20(3ms)%22%2C%22%232%20T027%22%2C%22%233%20T021%20(37ms)%22%5D%2C%22code%22%3A%22UX004%22%7D&csrf_token=YzQ0N3F3NXlTVzBVWTFraG96cmlVU3FVbjVNRmZRbHZ6RURIaHdPRExNRTlDRVRNL3hPN00xRXhoOUV0bnE0V3pYc3ZYcEg0YzRhVnp5WE5QYTJZN0E9PTsyMDE3LTA4LTA4VDAwOjU3OjM2Ljc3MjM1MDlaO283Mm9nSFVXb3lIbWtVZy9CeHZVbFE9PTt7Ik9yY2hlc3RyYXRpb25TdGVwIjoxfQ==&tx=eyJUSUQiOiI4MDgwNWE3Ny02OTU2LTRiNGMtYmUyYi05OGZkZGEwYzM4MDkifQ&desc=https%3A%2F%2Fourdomain.redacted.html

We have tested the following with no success:

• Clearing all our cache and cooking
• Disabled all extensions
• Private browsing/Incognito mode
• Chrome on Android

But Internet Explorer loads the custom sign-in page just fine on their computer!

I have tried searching online for error code UX004 but didn't find anything. Can someone from Microsoft advise what this error code means? Thank you.

Answer 1

I didn't get any response from Microsoft, but we tried various fixes. The one that worked for us is to apply a SSL certificate issued by a commonly-trusted issuer on our test domain. I can't confirm that this error message means AAD B2C is complaining of an insecure connection, but it's worth exploring in case it works for anyone else too.