Action 144, open data stream failed

Hi,

I am plagued with errors when I perform (differential) images to a NAS via ftp.
From time to time a backup fails with the errors "open data stream failed". As a rough estimate: 40% of all backups fail. This error occurs directly after the action 144 started, as can be seen from the log (see attachment). Most or all of the backup was completed by then. It seems to be the last action before finish.

Sometimes this can be resolved by deleting the failed backup and trying again (iterated several times). The error seems to occur randomly and is persistent for several months now. I usually set up a new backup script and start a new backup chain, which is quite tiresome.

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.

My questions are:
- What is action 144?
- Is this a general problem with Acronis or is it an error that I can resolve?
- What can I do to resolve the error?

Any help is greatly appreciated!