Details about the Linux host server running the Nextcloud AIO container, including how Nextcloud AIO accesses file storage, etc. would be helpful.
How is Syncthing installed? Also in a container?
It all depends on how the storage is set up for Nextcloud AIO. When working with containers, there’s usually a pool of shared storage. You’ll have to configure permissions on the storage so that both Nextcloud AIO and Syncthing can access it.