The old kernel version means you wouldn’t be able to run the ‘Landlock’ sandboxing. But that should degrade gracefully, and show a large warning in the admin panel.
The problem here seems to be the ‘seccomp’ protections, which we try to apply in all situations. If the synology kernel is missing seccomp (which sounds likely), then that explains the failure.
We might be able to make it fail more gracefully, but then your forum’s image processing would be running with absolutely no sandboxing.
So either way, I’d strongly recommend trying to find a more up-to-date/secure environment to run your forum, especially if it’s on the public internet. Image processing is becoming an increasingly common vector for exploits in recent months, which is why we’ve added the sandboxing.