Again, that looks fine…
You might want to wait a few days till Virtualmin 3.96 comes out, as I have fixed some bugs in this area … it’s possible that the new version will solve this problem.
Lovely! Much appreciated, look forward to that version. 
Success!
Reporting that the remote files were deleted with Webmin 3.96!
The backup failed though because the disk was full already (I was lazy to delete it), so the upload failed, but from tomorrow it will be working fine as there is enough space now…Many thanks.
Uploading archive to FTP server xxxxxxx.xxxxxxx.de …
… upload failed! STOR /backup/28-11-2012/mysitename-m.com.tar.gz failed : /backup/28-11-2012/nysitename-m.com.tar.gz: No such file or directory
Deleting backups from /backup/%d-%m-%Y on FTP server xxxxxx.xxxxxx.de older than 2 days …
Deleting FTP file /backup/19-11-2012, which is 9 days old …
… deleted 28.49 GB.
Deleting FTP file /backup/20-11-2012, which is 8 days old ..
.. deleted 28.73 GB.
Deleting FTP file /backup/21-11-2012, which is 7 days old ..
.. deleted 28.96 GB.
Deleting FTP file /backup/22-11-2012, which is 6 days old ..
.. deleted 13.81 GB.
… deleted 4 old backups