SharePoint 2010 backup error with InvalidData

Question

The same command ran well just a few days ago. I can't remember what have been done on the server.

PS C:> Backup-SPSite -Identity http://localhost - Path C:\backup.bak
Backup-SPSite : Operation is not valid due to the current state of the object.
At line:1 char:14
+ Backup-SPSite <<<<  -Identity http://localhost -Path C:\backup.bak
    + CategoryInfo          : InvalidData: (Microsoft.Share...mdletBackupSite:
   SPCmdletBackupSite) [Backup-SPSite], InvalidOperationException
    + FullyQualifiedErrorId : Microsoft.SharePoint.PowerShell.SPCmdletBackupSi
   te

Backup-SPSite : Operation is not valid due to the current state of the object.
At line:1 char:14
+ Backup-SPSite <<<<  -Identity http://localhost -Path C:\backup.bak
    + CategoryInfo          : NotSpecified: (:) [Backup-SPSite], InvalidOperat
   ionException
    + FullyQualifiedErrorId : System.InvalidOperationException,Microsoft.Share
   Point.PowerShell.SPCmdletBackupSite