If you have already configured the Portal to use Azure AD B2C but still getting error; something like shown below then follow along to know how to debug and resolve it.
There could be several reasons and lets understand what happens behind the scene that will help debug the problem.
When a new user signs up a user record is created in Azure AD as well as a contact record gets created in CRM and Azure creates an object id which is stored as Portal Username on the Contact record in CRM.
Possible Issue #1
Contact creation in CRM is failing due to existing plugin or a workflow or both. First thing disable all plugins and workflows. Then try signing up. If it works then you know it is one of your plugin or workflow and concentrate on identifying which one.
You can also open a ticket with Microsoft support and they might tell you which workflow is actually causing this issue.
Possible Issue #2
Now, if after disabling all your plugins the signup still threw an error then this might be the possible cause of failure.
The length of the attribute ‘adx_identityprovidername’ of entity ‘adx_externalidentity’ exceeded the maximum allowed length of ‘100’.
To resolve this error, increase the length of the attribute. I had it set to 300.
If none of the above resolution resolved your issues then opening up a case with Microsoft support would be the best option. Also if you do get it to resolved; let me know what you did to resolve it so it can help others who run into same issues.
Possible Issue #3
If you are getting error saying “Sign in failed” then add your portal base URL in the Reply URL on the Azure AD Application. You can see more details in the Fiddler or using Browser DevTools (or Developer Toolkit).
This post will be kept updated as more issues are encountered.