every 2nd rsync backup fails with @ERROR: chroot failed

Hi,

I have a Readynas NV+ v1 and have setup various backup jobs to the same rsync server. Each backups a subset of the NAS.

I don't know exactly when or why it started but since some weeks I have big problems with these rsync backups. As explained below I found out that every second backup job fails with this error:
@ERROR: chroot failed
rsync error: error starting client-server protocol (code 5) at main.c(1534) [sender=3.0.9]

And it's really strange, every 2nd job fails. So I SSHed into the box and tried the rsync command (which I got from "top") manually in the shell:
rsync -aAv --links -8 --delete --compress --port=873 . ftpbackup-217075@rsync1.euserv.de::ftpbackup-217075/ReadyNAS/MediaVault/Install

and what I twanted to backup is /c/Install, so I cd'ed into this directory and issued the command above. And the error occurs really alternating. Means once the command runs fine. Then I execute it a 2nd time from the shell history and it fails with the above error. the 3rd time it runs fine, the 4th time it fails with same error and so one.

I already tried modifying /etc/frontview/Shares.conf to not use chroot with "use chroot = no" and instead "fake super = yes" but with no luck. I also tried experimenting with the rsync command by setting timeouts or limiting connections to only 1 but with no luck so far.

Does anyone know what this bevaviour could be because also /var/log/rsync.log is not very helpful, there I see:
2012/09/08 21:03:07 [1379] rsyncd version 3.0.9 starting, listening on port 873
2012/09/30 14:32:11 [1390] rsyncd version 3.0.9 starting, listening on port 873
2012/10/05 14:09:10 [1390] rsync error: received SIGINT, SIGTERM, or SIGHUP (code 20) at rsync.c(551) [Receiver=3.0.9]
2012/10/05 14:18:04 [1744] rsyncd version 3.0.9 starting, listening on port 873
2013/02/28 13:47:05 [1417] rsyncd version 3.0.9 starting, listening on port 873
2013/05/18 12:58:22 [1417] rsync error: received SIGINT, SIGTERM, or SIGHUP (code 20) at rsync.c(551) [Receiver=3.0.9]

As you can see here from the log too, once it works once not. For example the job with the pid 1379 worked, but 1390 had the error, 1744 worked, 1417 had the error and so on.

Also rebooting did not help, I'm still getting the same errors. I'm really out of ideas. I really hope anyone can help. I'm also not sure if this started with the RAIDiator update to 4.1.13. But I don't know where to get an old version to test and if it's safe to downgrade.

Thanks for any help on this.