How to fix Error 3245 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 3245 – “Could Not Convert a String to or from Unicode”

Why This Error Happens

Error 3245 typically occurs during database backup or restore operations when SQL Server encounters a corrupted media header or an incompatible code page while attempting to translate string data between Unicode and non-Unicode formats. This is often caused by a damaged backup file, an incomplete file transfer (especially across different OS locales), or a mismatch between the collation settings of the source and destination servers. In some cases, the backup media itself may have been truncated or corrupted during storage or network transfer.

Step-by-Step Resolution Guide

Step 1: Verify Backup File Integrity

  • Check the backup file size against the expected size from the source server to rule out truncation.
  • Run RESTORE VERIFYONLY FROM DISK = 'C:\Backups\YourFile.bak' in SQL Server Management Studio (SSMS) to test if the header is readable.
  • If the command fails immediately, the file is likely corrupted and will need to be re-generated from the source.

Step 2: Check Server and Database Collation Settings

  • Open SSMS and run SELECT SERVERPROPERTY('Collation'); on both the source and destination instances.
  • Ensure both Windows Server 2025 instances are using compatible Unicode collations (e.g., SQL_Latin1_General_CP1_CI_AS).
  • If a mismatch exists, use the COLLATE clause in your query or rebuild the database with the matching collation before restoring.

Step 3: Re-transfer the Backup Using Binary Mode

  • If the file was moved via FTP or a network share, ensure the transfer protocol used Binary mode rather than ASCII mode, as ASCII mode corrupts Unicode byte structures.
  • Navigate to the file location (e.g., D:\MSSQL\Backup\) and verify permissions allow the SQL Server service account full read/write access.
  • Re-copy the file using Robocopy with the /J (unbuffered I/O) flag to prevent Windows Server 2025 caching issues: robocopy C:\Source D:\Destination YourFile.bak /J.

Step 4: Update SQL Server 2025 Cumulative Updates

  • Navigate to the Microsoft Update Catalog and check if a specific Cumulative Update (CU) addresses Unicode conversion bugs for your build.
  • Apply the update via the SQL Server Installation Center, selecting Maintenance > Edition Upgrade/Patch.
  • Restart the SQL Server (MSSQLSERVER) service via services.msc after installation completes.

Alternative or Verification Steps

  • Verification: Attempt the RESTORE DATABASE command again after applying the fixes; success is confirmed if the database mounts and appears “Online” in SSMS Object Explorer without rollback errors.
  • Alternative: If the error persists, attempt to restore the backup on a different intermediary server with a default SQL_Latin1_General_CP1_CI_AS collation to isolate whether the issue is collation-specific or purely file corruption.