Solved 
I switched from attaching the external storage from ftp to ssh and mounted the storage via sshfs. This seems to work around the issue. Backup is now functioning
Shiny!!! Thanks for GhettoVCB
EDIT: just a side note, using sshfs over curlftpfs has some impact on speed. Especially when restoring. I'm looking at some tweaks maybe someone else has some input to this setup.