I've received the same error one time, for a handful of databases, performing native SQL Server Transaction Log Backups, followed by error 53 a second later. Then backup operations to the same NAS location began working again.

Transaction Logs backup every 15 minutes. Each job is the standard MS SQL Agent Log Shipping Backup, created by the wizard.

One database on the server failed with the same error about 9 hours later.

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...Completed successfully...

11/29/2011 02:30:07,Backup,Unknown,Log was backed up. Database: !!!Database1!!!<c/> creation date(time): 2011/07/30(23:27:04)<c/> first LSN: 991365:14418:1<c/> last LSN: 991365:14418:1<c/> number of dump devices: 1<c/> device information: (FILE=1<c/> TYPE=DISK: {'\\den-sqldump\prod\den-sql3\Tran\!!!Database1!!!_20111129093007.trn'}). This is an informational message only. No user action is required.

11/29/2011 02:30:07,Backup,Unknown,Log was backed up. Database: !!!Database6!!!<c/> creation date(time): 2011/07/31(00:02:54)<c/> first LSN: 90691:15669:1<c/> last LSN: 90691:15669:1<c/> number of dump devices: 1<c/> device information: (FILE=1<c/> TYPE=DISK: {'\\den-sqldump\prod\den-sql3\Tran\!!!Database6!!!_20111129093007.trn'}). This is an informational message only. No user action is required.

...Failed Transaction Log Backups...

11/29/2011 02:29:07,Backup,Unknown,BACKUP failed to complete the command BACKUP LOG !!!Database2!!!. Check the backup application log for detailed messages.

11/29/2011 02:29:07,Backup,Unknown,Error: 3041<c/> Severity: 16<c/> State: 1.

11/29/2011 02:29:07,Backup,Unknown,BACKUP failed to complete the command BACKUP LOG !!!Database3!!!. Check the backup application log for detailed messages.

11/29/2011 02:29:07,Backup,Unknown,Error: 3041<c/> Severity: 16<c/> State: 1.

11/29/2011 02:29:07,spid75,Unknown,BackupDiskFile::CreateMedia: Backup device '\\den-sqldump\prod\den-sql3\Tran\!!!Database2!!!_20111129092907.trn' failed to create. Operating system error 53(The network path was not found.).

11/29/2011 02:29:07,spid75,Unknown,Error: 18204<c/> Severity: 16<c/> State: 1.

11/29/2011 02:29:07,spid74,Unknown,BackupDiskFile::CreateMedia: Backup device '\\den-sqldump\prod\den-sql3\Tran\!!!Database3!!!_20111129092907.trn' failed to create. Operating system error 53(The network path was not found.).

11/29/2011 02:29:07,spid74,Unknown,Error: 18204<c/> Severity: 16<c/> State: 1.

11/29/2011 02:28:08,Backup,Unknown,BACKUP failed to complete the command BACKUP LOG !!!Database4!!!. Check the backup application log for detailed messages.

11/29/2011 02:28:08,Backup,Unknown,Error: 3041<c/> Severity: 16<c/> State: 1.

11/29/2011 02:28:08,spid71,Unknown,BackupDiskFile::CreateMedia: Backup device '\\den-sqldump\prod\den-sql3\Tran\!!!Database4!!!_20111129092808.trn' failed to create. Operating system error 87(The parameter is incorrect.).

11/29/2011 02:28:08,spid71,Unknown,Error: 18204<c/> Severity: 16<c/> State: 1.

11/29/2011 02:28:07,Backup,Unknown,BACKUP failed to complete the command BACKUP LOG !!!Database5!!!. Check the backup application log for detailed messages.

11/29/2011 02:28:07,Backup,Unknown,Error: 3041<c/> Severity: 16<c/> State: 1.

11/29/2011 02:28:07,spid65,Unknown,BackupDiskFile::CreateMedia: Backup device '\\den-sqldump\prod\den-sql3\Tran\!!!Database5!!!_20111129092807.trn' failed to create. Operating system error 87(The parameter is incorrect.).

11/29/2011 02:28:07,spid65,Unknown,Error: 18204<c/> Severity: 16<c/> State: 1.