backup error, ftp to NAS, ATIH 2011

My backup plan from my hard drive to a network access storage fails repeatedly when the next full backup is to be made. I use differential backups and make a full version after every 5 differential backups. Not more than 2 version chains are to be kept. When the second full backup is to be made it seems as if it runs through. But short before the end I receive the error message that the operation 144 (saving volume structure) failed. What can be the cause of that?

My version of Acronis is: ATIH 2011, Update 3, Build 6942
Operating system: Windows 7, 64bit

The NAS drive seems to function properly (Synology diskstation 1511+, firmware 4.0), the connection is via ethernet and uses ftp as protocol.

Please note that the error mesaages are my translations from the original error messages in German. Attached is a text file of the log of a failed backup.

Any help greatly appreciated!