This error indicates that the backup destination is having trouble accessing the backup archive for your computer. This is not a connectivity problem between the two machines; as long as there is a green bubble next to the destination's name, then the client is connected to the destination computer.
The error message is displayed to let you know that backup cannot continue, but the actual error is occurring on the target machine and that's where you should look to resolve this situation.
On the target machine you'll want to check the path to the backup archive and make sure it's available and that all disks are properly mounted.
If the target is a PRO Server, try How to troubleshoot a client that is not backing up
Volume shadow service may be stopping the service - Unselect the “Backup Open Files” option.