Backup to NAS

I am using Acronis on a Windows 7 PC for several years now. I have been backing up files to an external NAS that is connected to my PC via a Gbit switch. I have been doing incremental updates and have been saving many past backups over time.

In the last year or so, perhaps with the latest version, I started having problems. The backup will work find for days, sometimes weeks, then decide to stop working with the following error:

DETAILS: Failed to prepare data stream. Check whether the source and target partitions exist.
ERROR CODE: More information about this error and solutions may be available online in the Acronis Knowledge Base. To access the online resource manually, enter the event code at: http://kb.acronis.com/errorcode/
Event code: 0x000B03E9+0x0004000F+0x00040007+0x0000FFF0+0x80070003

I have researched this error, but gotten nowhere. The NAS is always available and the backup executes during down time in the middle of the night. When trying to run the backup during the next morning, when I see that the NAS is available and I can access it from my PC without a problem, the backup will not work. Actually, it will never work again until I create a brand new backup. THEN it will work!!!

If I do nothing, on the next and subsequent days I get a different error:

Backup2014 Task is waiting for user interaction.
Description: Stage Description
Information: Error occurred while opening the file.
Details: Please insert the last media of the archive and click Retry to continue. Otherwise click Cancel to stop the operation.

Now that is another error that I don't understand. Now it lost communication with the NAS completely and cannot even find the previous backup files? And why does it work if I setup another backup?

Any explanations would be appreciated.