Sorry it took me a bit to get my test environment set up again.
I talked to the team members and they let me know that everything resides inside "/sync" inside the container, which is by default mapped to "/storage/.kodi/userdata/addon_data/docker.linuxserver.resilio-sync/sync" on libreelec. But you can change that in the addon settings.
The resilio-sync client runs as user nobody (65534) so all the files in that sync folder should be owned by that.
I set up a fresh resilio-sync addon on my libreelec vm and I can go to add folder (standard), hit "new folder" and create one. All those newly created folders are under "/sync". I didn't have any issues.
The addon creates those folders and changes ownership to user nobody on start.
See the attached screenshots.
Can you describe your environment a bit more? By the way, don't manually edit the settings.xml or the sync.conf. They are all managed by the addon and they'll break it completely.
EDIT: Did you by any chance select a folder on a remote mount as the sync location? If the remote mount didn't allow chowning that location as user nobody, that would be the issue.