I had this same issue. Followed the error code, and just used command line updater. In that NAS, you’ll need to find a way to access the container’s terminal.
If whatever interface you’re using brings you into the system terminal you’ll need to enter the container’s. Run docker ps and find the CONTAINER ID of nextcloud. (I.e. mine is 967653…)
If it says “command not found” then you’re probably inside the container, skip the docker part
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
9676532d0b29 nextcloud:29 "/entrypoint.sh apac…" 12 minutes ago Up 12 minutes 0.0.0.0:9520->80/tcp, [::]:9520->80/tcp nextcloud-app
afb4487fb53a redis:latest "docker-entrypoint.s…" 12 minutes ago Up 12 minutes 0/tcp, 6379/tcp nextcloud-redis
de85a93c181d mariadb:lts "docker-entrypoint.s…" 12 minutes ago Up 12 minutes 3306/tcp nextcloud-db
If this doesn’t error, then that means you’re in the system terminal, so run this to enter the container’s terminal:
docker exec -it -u 33 9676532d0b29 /bin/bash
(Replacing that long number with your id)
Then, once you’re in, type pwd and make sure you’re in the /var/www/html directory. If not, run cd /var/www/html. Then run:
./occ upgrade
and you should see an “in progress” message (wait a bit)
(If you run ./occ in the future, you get access to some other helpful commands for managing your nextcloud instance)
Hope this helps, if you get stuck anywhere don’t hesitate to send a reply!