Can I manually reset the "internal status flag for a full database backup"? We recently switched to the FULL recovery model, with full backups being done every weekend, and differentials being taken every day.

This worked last weekend, however this weekend the differential backup failed saying that


  Message
  
  Executed as user: username. ... 9.00.3042.00 for 32-bit 
  Copyright (C) Microsoft Corp 1984-2005. All rights reserved.
  Started:  12:30:00 AM
  Progress: 2013-01-21 00:30:01.69
  Source:
  {450389BA-54C2-4892-9CD0-0126CA9B0ED8}      Executing query "DECLARE
  @Guid UNIQUEIDENTIFIER      EXECUTE msdb..sp".: 100% complete
  End
  Progress
  Error: 2013-01-21 00:30:02.11     Code: 0xC002F210
  Source: Back Up Database (Differential) Execute SQL Task
  Description: Executing the query "BACKUP DATABASE [MyDatabase] TO 
  DISK = N'E:\Database Backups\MyDatabase_backup_201301210030.bak' WITH
  DIFFERENTIAL , NOFORMAT, NOINIT,  NAME =
  N'MyDatabase_backup_20130121003002', SKIP, REWIND, NOUNLOAD,  STATS =
  10  " failed with the following error: "Cannot perform a differential
  backup for database "MyDatabase", because a current database backup
  does not exist. Perform a full database backup by reissuing BACKUP
  DATABASE, omitting the WITH DIFFERENTIAL option.   BAC...  The package
  execution fa...  The step failed.


The job that runs the full database backup ran successfully, and I can view the backup using the query found here, so I am not sure why it is telling me "a current database backup does not exist".

I found this MSDN page which suggests that the cause of the problem is 


  This problem may occur when the internal status flag for a full
  database backup does not reset after you create the full database
  backup.


I'm not able to install the hotfix right away, and doing another full database backup is not an option right now, so I was looking to try manually resetting the internal status flag for a full database backup.

Is there a way to manually reset this flag so that differentials can successfully be ran for the rest of the week? Or to set the base backup to an existing full backup that exists in msdb.dbo.BackupSet/msdb.dbo.BackupMediaFamily?

I'm using SQL Server 2005
Your primary goal here is to ensure your ability to restore the database.  As long as you have the complete chain of your log backups from the last full to now, you can perform a point in time recovery and the error is merely an inconvenience.  Differential backups are not required for full point in time recovery, just provide you with a shortcut for larger time frames.  However, in the absence of your differentials, you'll need all the log backups to cover the gap.

Otherwise, the only way to properly reset this is to execute another actual full backup.  I would recommend executing this as soon as possible which, depending on the size of your database, should be something that could be accomplished overnight in place of your normal differential.

Also, I would strongly recommend that you apply Service Pack 4 for SQL 2005 to make sure you have the most current hotfixes/updates for your platform.