Hi
@Helen: Thanks, that is very considered of you.
Resilio Sync Support's answer pushed me in the right direction.
The problem seems to be, that as the partial downloads of a file are saved in .sync folder and are moved to there correct location after download completion... this file-move action is not supported cross-partition (meaning .sync folder lies on a different partition as the resulting location of the file).
A workaround seems to be 'mhddfs'.
mhddfs combines several disks/mountpoints into one folder/virtual file system. With this, moving the files from .sync folder does not seem to be a cross-partition action for Resilio Sync anymore, since apparently mhddfs (or the underlying fuse) takes care of the cross-partition action.
This is my working line in /etc/fstab:
mhddfs#/mnt/disk1,/mnt/disk2 /project fuse defaults,allow_other,nonempty,default_permissions 0 0
Thanks to Support for the help.
Kind regards
Oli4