Errors when rsync with Synology

[Max-Hipshot]

f+++++++ Archief/BVV 1986/Scanned Image 72.tiff
32768 1% 34.22kB/s 0:00:47
1376256 82% 1.31MB/s 0:00:00
1671020 100% 1.58MB/s 0:00:01 (xfer#159, to-check=103301/103610)
send_files failed to open "/tmp/.._Scanned Image 72.tiff.yiDWdM": Operation not permitted

This occurs on all files in the source.
The source is the original built-in HDD spindle in my imac, that I keep as archive drive. I want to rsync it with an AFP fileshare on my synology drive station.

I have granted grsyncx full access to disks as per your recommendation on your support page.

Questions:
What causes the permission error
which /tmp/ directory are we talking here? Local on macos or at the destination location on my synology drive?
Also, I have rsync service running on the synology. Can't I fire up against the Synology host directly instead of