File I/O Error after successfull incremental Backup

Dear expert users
I do an incremental backup of a whole HDD to a buffalo NAS device over a LAN.
An external USB HDD is attached at the buffalo device.

I got the "Acronis True Image Home Notification von Server-Susi" by email.
The backup and even the validation where successfull.
The backup files are on a buffalo NAS device within the network.
First question: Is the validation over the complet backup including the first full backup?

15min later I received following email:
"Backup_Server-Susi wartet auf einen Benutzereingriff.
Beschreibung: Festplatten-Backup
Information: Datei I/O Fehler.
Details: Legen Sie das letzte Volumen des Backup-Archivs ein und klicken Sie auf Wiederholen, um fortzusetzen. Andernfalls klicken Sie auf Abbrechen, um die Aktion abzubrechen."
Translation:
"Backup_Server-Susi is waiting for a userinput.
Description: Harddiskdrive-Backup
Information: File I/O Error.
Details: Insert the last volume of the backup-archive and click retry to go ahead or abort to stop the operation"

Now, according Acronis Task, the backup of this day was not successfull eventhough the first email confirms it. What's the cause for this I/O Error? It never happens with TIH 2009.

Thank you very much your for efforts, writing me the answers.
Best wishes.