How to fix Error 3968 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 3968 – Snapshot Isolation Unavailable Due to Undocumented Trace Flags

Why This Error Happens

This error occurs because SQL Server 2025 was started with one or more undocumented (internal/diagnostic) trace flags that explicitly disable row versioning support at the engine level. Row versioning is a prerequisite for both snapshot isolation and read committed snapshot isolation (RCSI), and certain internal trace flags—often used by Microsoft support for diagnostic purposes—block this subsystem from initializing. As a result, any transaction attempting to use SNAPSHOT isolation or any database with RCSI enabled will fail because the versioning store cannot be created.

Step-by-Step Resolution Guide

Step 1: Identify Active Trace Flags on the Instance

  • Open SQL Server Management Studio (SSMS) and connect to the affected instance.
  • Run DBCC TRACESTATUS(-1); to list all globally enabled trace flags.
  • Cross-reference the output against known documented trace flags; any flag not listed in official Microsoft documentation is a candidate for removal.
  • Alternatively, check the SQL Server Error Log (C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG) for a line listing startup trace flags.

Step 2: Inspect SQL Server Startup Parameters

  • Open SQL Server Configuration Manager from the Start menu on Windows Server 2025 / Windows 11.
  • Navigate to SQL Server Services > right-click your instance (e.g., SQL Server (MSSQLSERVER)) > Properties.
  • Go to the Startup Parameters tab and review all entries prefixed with -T (these represent trace flags passed at boot).
  • Note down any undocumented flag numbers (commonly internal flags outside the publicly documented range) for removal.

Step 3: Remove the Undocumented Trace Flags

  • In the Startup Parameters tab, select the problematic -T<flag_number> entry and click Remove.
  • Click Apply, then restart the SQL Server service for changes to take effect.
  • If the flags were set via a Windows Registry edit or a startup script instead of Configuration Manager, locate and edit the registry key at:
    HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQLServer\Parameters
  • Delete the corresponding SQLArgn value containing the unwanted trace flag.

Step 4: Restart the SQL Server Service

  • Open Services.msc on Windows Server 2025 / Windows 11.
  • Locate SQL Server (MSSQLSERVER) (or named instance equivalent).
  • Right-click and select Restart to apply the startup parameter changes.
  • Confirm the service returns to a Running state without errors in the Windows Event Viewer.

Step 5: Re-enable Snapshot Isolation on the Database

  • Connect via SSMS and run the following T-SQL to enable the required isolation features:
  ALTER DATABASE [YourDatabaseName] SET ALLOW_SNAPSHOT_ISOLATION ON;
  ALTER DATABASE [YourDatabaseName] SET READ_COMMITTED_SNAPSHOT ON;
  • Ensure no active connections are blocking the ALTER DATABASE command; switch to single-user mode temporarily if needed.

Alternative or Verification Steps

  • Verify the fix by querying sys.databases with SELECT name, snapshot_isolation_state_desc, is_read_committed_snapshot_on FROM sys.databases WHERE name = 'YourDatabaseName';—both values should confirm the features are active.
  • If removing trace flags is not feasible (e.g., required by a support case), consider migrating the workload to a different instance without the conflicting startup parameters, or contact Microsoft Support to identify a safe equivalent documented flag.