Acronis cannot delete files via FTP

I am using Acronis true image home 2011 Update 3 Build 6942
System: Windows 7 64 bit
2 Disks: 1 SSD 128 GB + 1 HDD Raid1 morror 1TB.

Acronis is backing up the files properly but is not deleting older versions via FTP.

Detailled description:
I defined two task to fully backup two disks. with the option to delete files after 6 months. For the backup I use an FTP connection to my local NAS for security reasons.

Everything works fine exept the fact that acronis is not deleting older files. That causes the NAS to overflow.

When I go to acronis right click the backup task and select delete, I confirm to delete the job and the archives, The gob is gone but the archives are still there.

When I logon to the NAS with Filezilla with the same account I can delete files without any problem.

Does anybody have an idea what can cause this error?

Thanks in advance

Doc Alex