How to fix Error 4021 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 4021 — Connection Reset Results in Different Login State

Why This Error Happens

Error 4021 occurs when SQL Server’s connection resiliency feature attempts to reset a session after an idle or dropped connection, but the session-level settings (such as database context, SET options, or temporary objects) cannot be restored to match the original login state. This typically happens due to changes in default database availability, mismatched collation or language settings applied mid-session, or interference from connection pooling in the client driver (such as an outdated ODBC or OLE DB provider). It can also be triggered when Contained Database Users or Azure AD/Entra ID authentication tokens expire mid-session on Windows Server 2025 environments.

Step-by-Step Resolution Guide

Step 1: Verify the Default Database State

  • Open SQL Server Management Studio (SSMS) and run SELECT name, state_desc FROM sys.databases; to confirm the user’s default database is ONLINE and not RESTORING or OFFLINE.
  • Check sp_helplogin 'username' to ensure the default database assigned to the login still exists and hasn’t been renamed or dropped.
  • If the default database is unavailable, reassign it using ALTER LOGIN [username] WITH DEFAULT_DATABASE = [master];

Step 2: Update Client Drivers and Connection Strings

  • Navigate to Control Panel > Programs > Programs and Features on the Windows Server 2025/Windows 11 machine and check the version of “Microsoft ODBC Driver for SQL Server” or “Microsoft OLE DB Driver for SQL Server.”
  • Download and install the latest driver from the Microsoft Download Center if the version predates 18.x.
  • In the application’s connection string, disable aggressive connection pooling by setting Pooling=false; temporarily to isolate whether pooling is causing stale session resets.
  • Alternatively, add ConnectRetryCount=3;ConnectRetryInterval=10; to allow graceful retries instead of abrupt resets.

Step 3: Audit SET Options and Session Context Scripts

  • Open SQL Server Configuration Manager and review any Logon Triggers under Server Objects > Triggers in SSMS that might be altering session state (e.g., SET ANSI_NULLS, SET QUOTED_IDENTIFIER) inconsistently.
  • Run DBCC USEROPTIONS; immediately after connecting to compare active session settings against the expected defaults.
  • Disable any custom logon triggers temporarily using DISABLE TRIGGER [trigger_name] ON ALL SERVER; to test if they are the source of the state mismatch.

Step 4: Check Authentication Token Expiry (Azure AD/Entra ID)

  • If using Entra ID authentication, open Event Viewer on the Windows Server 2025 host and navigate to Applications and Services Logs > Microsoft > Windows > AAD.
  • Look for token expiration events that align with the timestamp of Error 4021.
  • Increase the token refresh interval in your application’s authentication library or switch to SQL Server native authentication for service accounts experiencing frequent resets.

Step 5: Restart SQL Server Services and Clear Connection Cache

  • Open Services.msc, locate “SQL Server (MSSQLSERVER)” or the named instance, and perform a restart to clear any cached session states.
  • Run DBCC FREESYSTEMCACHE('ALL'); in SSMS to purge stale connection and plan cache data that might be causing the mismatch.

Alternative or Verification Steps

  • To verify the fix, open a new connection in SSMS, run a test transaction that switches database context, then force a network interruption (e.g., briefly disable the network adapter) to confirm the connection resumes without triggering Error 4021.
  • If the issue persists, enable Extended Events with the error_reported event filtered for Error 4021 to capture the exact session state mismatch for deeper diagnosis with Microsoft Support.