In such cases, it’s not a issue of Syncthing.
To fix this again, I recommend resetting recursiv the Synology DSM-ACLs in this directory /volume1/Movies/ for all content with root:root, octal 0777 and then resetting the DSM-ACLs in the DSM file manager by inheriting them from the root directory /volume1/Movies/ back into the files and subdirectories. Then it should work again.