Backup failed: System.Data.SqlClient.SqlError: Backup and file manipulation... must be serialized

Using SQL Server 2005 Server Management Studio, I attempted to back up a database, and received this error:

Backup failed: System.Data.SqlClient.SqlError: Backup and file manipulation operations (such as ALTER DATABASE ADD FILE) on a database must be serialized. Reissue the satement after the current backup or file manipulation is completed (Microsoft.SqlServer.Smo)

Program location:

   at Microsoft.SqlServer.Management.Smo.Backup.SqlBackup(Server srv)
   at Microsoft.SqlServer.Management.SqlManagerUI.BackupPropOptions.OnRunNow(Object sender)

Backup Options were set to:

Back up to the existing media set

     Overwrite all existing backup sets

I am fairly new to SQL 2005.  Can someone help me get past this issue? What other information do I need to provide?