Failed Signup and Login with error Unable to construct login user using SAML enterprise connection

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I am having my own database connection and trying to add also enterprise connection with SAML. User is created in the DB and then after login with Microsoft Azure Account using SAML, I am linking both accounts. While the user is login with SAML connection, Unable to construct login user error is generate for both Failed Signup and Failed Login. Despite the error, Azure and DB accounts are linked. Account linking is made in post-login action.

Could you please help me with error, everything seems fine and I can’t find any information about this particular error?