Scheduled Backup to FTP location fails - Read Only - all other means of FTP connection can Write

I did setup a scheduled virtual server backup to an FTP location.

Location is a WNDR4500v2 attached storage setup as FTP
this is what it returns:

Starting backup of 1 domains to /ftp/folder/folder/Full%d-%m-%Y on FTP server

Creating backup for virtual server

Dumping MySQL database dbname ..
.. done

Creating ZIP file of home directory ..
.. done

Uploading archive to FTP server ..
.. upload failed! STOR /ftp/folder/folder/Full11-09-2015/ failed : Read Only

… completed in 4 seconds

Backup failed! See the progress output above for the reason why.

Now i checked the read only over and over… when I ftp using commandline , I can create and store files and folder, no problems.
When I use filezilla client, I can easily create and store files and folders, no problems
When using the scheduled backup featur in virtualmin… read only.

Only thing different: in filezilla it shows the root folder /shares/ which is not accessible /ftp is a subfolder of /shares/.

When using ftp commandline it starts at folder /ftp/ in the root, parent folder /shares/ does not show.
I run the commandline using shell on the virtualmin machine

Adding /shares/ in the “file on server” does not make a difference nor removing it…

I am puzzled… anyone can point me in the right direction?