Yes, it is set to send-only, and due to the problem with privileges, I cannot change that. But I do NOT want to delete the files from NAS! That’s why I originally had the unidirectional sync set with ignore-delete. Now I changed it to bidirectional sync, but again with ignore-delete. I want the photos synced from the phone to remain at NAS until I move them manually out. I do not want the deletion at the phone side (e.g. because of the SD card becomes full) to result in removal of the photos from NAS.
Of course, I can backup the photos for now and perform experiments, but only for a limited time. After resolving the problems I want to return to the described state.