Azure Ad B2C Error : "Unable to validate the information provided." when using custom policy and custom attribute - azure-ad-b2c

Question

I am trying to use custom policies of Azure Ad B2C because we have a complex flow of self-registration for external users.
I am trying to use custom attributes, and I followed the dedicated tutorial.
I keep getting the error "Unable to validate the information provided" nevertheless.
1 - I added a section for AAD-Common with the ids of the b2c-extensionregistered app.
2 - I created custom attributes using the Azure AD B2C UI in the dedicated section.
3 - I declared this attribute in the ClaimSchemas section.
4 - I added the attribute in the self-asserted sign-up technical profile.
5 - I added the attribute to be persisted as a persistent claim in technical profile validating the self-sign-up.
6 - I uploaded the extension file on my Azure AD B2C custom policy page.
Can you tell me what I do wrong?
Thanks!
I double-checked several times the tutorial against my work before posting here.
Here is my extension file:
<?xml version="1.0" encoding="utf-8" ?>
<TrustFrameworkPolicy xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
xmlns="http://schemas.microsoft.com/online/cpim/schemas/2013/06" PolicySchemaVersion="0.3.0.0" TenantId="wynsureformercer.onmicrosoft.com" PolicyId="B2C_1A_TrustFrameworkExtensions" PublicPolicyUri="http://mytenant.onmicrosoft.com/B2C_1A_TrustFrameworkExtensions">
<BasePolicy>
<TenantId>mytenant.onmicrosoft.com</TenantId>
<PolicyId>B2C_1A_TrustFrameworkLocalization</PolicyId>
</BasePolicy>
<BuildingBlocks>
<ClaimsSchema>
<ClaimType Id="custom_employee_id">
<DisplayName>Employee Id</DisplayName>
<DataType>string</DataType>
<UserHelpText/>
<UserInputType>TextBox</UserInputType>
</ClaimType>
</ClaimsSchema>
</BuildingBlocks>
<ClaimsProviders>
<ClaimsProvider>
<DisplayName>Azure Active Directory</DisplayName>
<TechnicalProfiles>
<TechnicalProfile Id="AAD-Common">
<Metadata>
<!--Insert b2c-extensions-app application ID here, for example: 11111111-1111-1111-1111-111111111111-->
<Item Key="ClientId">f8070b57-64ba-blablabla</Item>
<!--Insert b2c-extensions-app application ObjectId here, for example: 22222222-2222-2222-2222-222222222222-->
<Item Key="ApplicationObjectId">a6ae751f-7d74-blablabla</Item>
</Metadata>
</TechnicalProfile>
<TechnicalProfile Id="AAD-UserWriteUsingLogonEmail">
<Metadata>
<Item Key="Operation">Write</Item>
<Item Key="RaiseErrorIfClaimsPrincipalAlreadyExists">true</Item>
</Metadata>
<IncludeInSso>false</IncludeInSso>
<InputClaims>
<InputClaim ClaimTypeReferenceId="email" PartnerClaimType="signInNames.emailAddress" Required="true" />
</InputClaims>
<PersistedClaims>
<!-- Required claims -->
<PersistedClaim ClaimTypeReferenceId="email" PartnerClaimType="signInNames.emailAddress" />
<PersistedClaim ClaimTypeReferenceId="newPassword" PartnerClaimType="password" />
<PersistedClaim ClaimTypeReferenceId="displayName" DefaultValue="unknown" />
<PersistedClaim ClaimTypeReferenceId="passwordPolicies" DefaultValue="DisablePasswordExpiration" />
<!-- Optional claims. -->
<PersistedClaim ClaimTypeReferenceId="givenName" />
<PersistedClaim ClaimTypeReferenceId="surname" />
<PersistedClaim ClaimTypeReferenceId="custom_employee_id" />
</PersistedClaims>
<OutputClaims>
<OutputClaim ClaimTypeReferenceId="objectId" />
<OutputClaim ClaimTypeReferenceId="newUser" PartnerClaimType="newClaimsPrincipalCreated" />
<OutputClaim ClaimTypeReferenceId="authenticationSource" DefaultValue="localAccountAuthentication" />
<OutputClaim ClaimTypeReferenceId="userPrincipalName" />
<OutputClaim ClaimTypeReferenceId="signInNames.emailAddress" />
</OutputClaims>
<IncludeTechnicalProfile ReferenceId="AAD-Common" />
<UseTechnicalProfileForSessionManagement ReferenceId="SM-AAD" />
</TechnicalProfile>
</TechnicalProfiles>
</ClaimsProvider>
<ClaimsProvider>
<DisplayName>Local Account SignIn</DisplayName>
<TechnicalProfiles>
<TechnicalProfile Id="login-NonInteractive">
<Metadata>
<Item Key="client_id">ProxyIdentityExperienceFrameworkAppId</Item>
<Item Key="IdTokenAudience">IdentityExperienceFrameworkAppId</Item>
</Metadata>
<InputClaims>
<InputClaim ClaimTypeReferenceId="client_id" DefaultValue="3fb15547-7986-416b-b6ba-4d84bee6770a" />
<InputClaim ClaimTypeReferenceId="resource_id" PartnerClaimType="resource" DefaultValue="1215af09-c958-4d92-a356-b1316895ff31" />
</InputClaims>
</TechnicalProfile>
</TechnicalProfiles>
</ClaimsProvider>
<ClaimsProvider>
<DisplayName>Local Account</DisplayName>
<TechnicalProfiles>
<TechnicalProfile Id="LocalAccountSignUpWithLogonEmail">
<DisplayName>Email signup</DisplayName>
<Protocol Name="Proprietary" Handler="Web.TPEngine.Providers.SelfAssertedAttributeProvider, Web.TPEngine, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" />
<Metadata>
<Item Key="IpAddressClaimReferenceId">IpAddress</Item>
<Item Key="ContentDefinitionReferenceId">api.localaccountsignup</Item>
</Metadata>
<CryptographicKeys>
<Key Id="issuer_secret" StorageReferenceId="B2C_1A_TokenSigningKeyContainer" />
</CryptographicKeys>
<InputClaims>
<InputClaim ClaimTypeReferenceId="email" />
</InputClaims>
<OutputClaims>
<OutputClaim ClaimTypeReferenceId="objectId" />
<OutputClaim ClaimTypeReferenceId="email" PartnerClaimType="Verified.Email" Required="true" />
<OutputClaim ClaimTypeReferenceId="newPassword" Required="true" />
<OutputClaim ClaimTypeReferenceId="reenterPassword" Required="true" />
<OutputClaim ClaimTypeReferenceId="executed-SelfAsserted-Input" DefaultValue="true" />
<OutputClaim ClaimTypeReferenceId="authenticationSource" />
<OutputClaim ClaimTypeReferenceId="newUser" />
<!-- Optional claims, to be collected from the user -->
<OutputClaim ClaimTypeReferenceId="displayName" />
<OutputClaim ClaimTypeReferenceId="givenName" />
<OutputClaim ClaimTypeReferenceId="surName" />
<OutputClaim ClaimTypeReferenceId="custom_employee_id" />
</OutputClaims>
<ValidationTechnicalProfiles>
<ValidationTechnicalProfile ReferenceId="AAD-UserWriteUsingLogonEmail" />
</ValidationTechnicalProfiles>
<UseTechnicalProfileForSessionManagement ReferenceId="SM-AAD" />
</TechnicalProfile>
</TechnicalProfiles>
</ClaimsProvider>
</ClaimsProviders>
<!--UserJourneys>
</UserJourneys-->
</TrustFrameworkPolicy>

Answer 1

When using a custom attribute in custom policies, you must prefix the claim type ID with extension_ to allow the correct data mapping to take place within the Azure AD B2C directory.
Also, make sure to copy the Object ID of the b2c-extensions-app in your portal to ApplicationObjectId and Application (client) ID of b2c-extensions-app to the client id in your custom policy correctly.