Damn. It was worth a shot. There might be a way to turn on more logging, but it's probably not worth the effort diagnosing the issue further. This far into the weeds it won't get you much, unless you want to start making bug reports to NextCloud

Yes, when you load the page. The logs for Docker itself would be viewed through SSH using something like journalctl -u docker.service. If you're lucky it'll report something there when you load the page and it makes the invalid request

I ssh into the Synology and run docker version, then read out the "API version". Is that right?

Yes, API version, specifically the server one.

Running inside Docker is great, except where your program also talks to Docker, in which case the host version can matter. I'm just guessing here, since the log doesn't actually list the url that got a 404, but it would fit the symptoms. You might be able to see something from the Docker daemon logs after opening the AIO web UI.

Interesting, my Synology only has 1.41 as the highest Docker API. It also has the standard /var/run/docker.sock path, no need for the override.

I don't run Nextcloud on it though. Synology's stuff is often too out of date or customised to run on the bare metal. Just getting Nextcloud backups working to the Synology with Borg was a nightmare with their weird SSH permissions system.

Judging by that error in the Master container logs and the incomplete status info, it might be failing to get the container status. Glancing at the AOI code, GuzzleHttp is what it uses to talk to Docker, so a 404 there suggests its making a request to docker but that url doesn't exist. My guess is it's your API version isn't actually supported by the AIO code - its asking for something that doesn't exist in 1.43.

Unfortunately, it looks like AIO recently (in the last few weeks) updated and the minimum Docker API is now 1.44, hence that warning you get. I would maybe try 1.41, as that was the previously supported version and is likely to be the most compatible. It doesn't seem possible to downgrade the AIO container either, as that will lock you to the associated NextCloud version.

I would consider an alternative installation method, tbh. A VM on the Synology might work, and let you update Docker independent of Synology's schedule