It's possible you can start by changing LoginName in the audit table to allow NULL , since this may not be available the way you're pulling from the XML in the case of SQL authentication logins. (And in fact I don't think you necessarily need the heavy EventData() XML at all.)

Next, you need to make sure your audit table supports the data that you might try to insert - login names, program names, and host names can all be up to nvarchar(128) , so that's how your columns and variable declarations should be declared.

Though, I don't think it needs both ClientHost and HostName since I don't know when those could be different.

Then hopefully you can log any error messages somewhere else.

ALTER TRIGGER [LogonAuditTrigger]
ON ALL SERVER 
FOR LOGON
AS
BEGIN
  DECLARE @EventTime  datetime      = getutcdate(),
          @LoginName  nvarchar(128) = ORIGINAL_LOGIN(),
          @HostName   nvarchar(128) = HOST_NAME(),
          @AppName    nvarchar(128) = APP_NAME();

  BEGIN TRY
    INSERT LogonAudit.dbo.LogonAuditing
    (
      SessionId,
      LogonTime,
      HostName,
      ProgramName,
      LoginName
    )
    SELECT
      @@spid,
      @EventTime,
      @HostName,
      @AppName,
      @LoginName;
  END TRY
  BEGIN CATCH
    INSERT dbo.LoggingTable ... stuff including ERROR_MESSAGE() ...
    THROW();
  END CATCH
END

The only other problem would be if the grafana user doesn't have the ability to write to the audit table (or the logging table). Try to insert a row manually using EXECUTE AS .