How to fix Error 1435 on Windows Server 2025 / Windows 11

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Why This Error Happens

Error 1435 occurs when a database mirroring endpoint receives an unexpected status response, typically because the mirroring session state does not match what the partner server expects (e.g., the principal or mirror was restarted, failed over, or manually altered outside of the mirroring context). This is commonly triggered by network interruptions between endpoints, expired or mismatched certificates used for endpoint authentication, or a witness server that has lost quorum communication with one of the partners. In Windows Server 2025 environments, stricter TLS 1.3 enforcement and updated firewall defaults can also cause endpoint handshake failures that manifest as this status mismatch.

Step-by-Step Resolution Guide

Step 1: Verify Endpoint State and Connectivity

  • Run SELECT * FROM sys.database_mirroring_endpoints; on both principal and mirror instances to confirm the endpoint state_desc is STARTED.
  • Test raw TCP connectivity using Test-NetConnection -ComputerName <PartnerServer> -Port 5022 from PowerShell.
  • Check Windows Defender Firewall rules on both servers to ensure the mirroring port (default 5022) is allowed for both inbound and outbound traffic.

Step 2: Validate Certificates and Authentication

  • Open SQL Server Configuration Manager and confirm the service account has not changed since the endpoint was created.
  • Run SELECT name, expiry_date FROM sys.certificates; to check for expired certificates used in endpoint authentication.
  • If certificates are expired or mismatched, recreate them using CREATE CERTIFICATE and reissue them to both partner instances, then re-run ALTER ENDPOINT to bind the new certificate.

Step 3: Restart the Mirroring Session

  • Pause mirroring using ALTER DATABASE <DBName> SET PARTNER SUSPEND; on the principal.
  • Resume the session with ALTER DATABASE <DBName> SET PARTNER RESUME; once connectivity and certificates are confirmed.
  • Review the SQL Server Error Log (Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG) for confirmation that the mirroring session has re-established without further 1435 entries.

Step 4: Check Witness Server Health (If Configured)

  • Confirm the witness instance is reachable and its endpoint is also in a STARTED state.
  • Validate that the witness server account has the correct CONNECT permission on the mirroring endpoint using GRANT CONNECT ON ENDPOINT::<EndpointName> TO [DOMAIN\ServiceAccount];.

Alternative or Verification Steps

  • Query sys.dm_db_mirroring_connections on both partners to confirm active, healthy connections with no repeated disconnect/reconnect cycles.
  • If the error persists after certificate and connectivity fixes, drop and recreate the mirroring endpoints entirely using DROP ENDPOINT followed by a fresh CREATE ENDPOINT configuration on both the principal and mirror servers.