This is a known issue by SAP Analytic.
2649833 - HTTP Status 400 - Bad Request after login to SAP Analytics Cloud
You enable a custom SAML Identity Provider in your SAP Analytics Cloud (SAC)
SAP Analytics Cloud •
Any valid SAML 2 Identity Provider (IdP) •
The SAML assertion returned to SAC doesn't contain a valid Name ID required to validate the user.
For example, you selected Custom SAML attribute as the attribute method to map users.
Value returned by the IdP:
Value in the column Custom SAML attribute:
Not only the values are different, also the nameid-format is expecting an email address.
Make sure that the value returned from your custom IdP matches the value selected in SAP Analytics Cloud.