Hi Manu,
I found several threads about this, that one led me on the right track:
http://forum.proxmox.com/threads/15341-Can-t-perform-backup
I found there was a bugfix:
https://goo.gl/uzfWIW
The forumthread speaks of also editing /usr/share/pve-manager/ext4/pvemanagerlib.js but in the other...