This error Login failed for SP User_2023 can be shown when logging into the SurePassID Authentication Server portal when there is an issue preventing the SPAS (MFA) server application from signing into the SQL instance/DB for the system.
This is often due to the SQL instance not allowing SQL Authentication and thus the SPUSER_20xx user account created by the installer is not allowed to sign into the SQL instance to access the DB.
The quick fix is to use SQL Management Studio to change the authentication options for the SQL instance to allow "mixed mode" authentication (both Windows and SQL authentication).
The recommended configuration is to use a managed service account (MSA) or a group managed service account (gMSA) for the access to the SQL instance/DB, changing the connection settings to match as noted in this KB article: What is a SurePassID Application (API) Key?
There is a chance that the password in the SQL instance and the password in the SPAS web.config file don't match, so if the SQL instance already has mixed mode enabled and the SPUSER_20xx has dbadmin access to the SurePassID20XX DB, then verify hat the password for the account in SQL is the same as the password in the connection settings in the web.config.