Hi Donnerknall,
sounds like the user running the container has no write access to the mounted folder. I think you have to give the container a user and group-id with suitable access rights.
There are two environment variables to set in the container settings (advances settings), PGID for the group-id and PUID for the user-id.
If I remember correctly, the easiest way to get your id’s is to ssh into your NAS and use the command id <user>.
There is a hint to these variables in the syncthing-docker-readme