That message does not explain why the system would stop responding. It means that the cache has fallen below the minimum free space value allowed to write a new file to the cache, so the file will be written directly to the array instead. You have 100GB set as the minimum free for the cache so that is the value probably triggering it.
I see that you have macvlan networking enabled for docker, but have not disabled bridging on eth0 as is recommended in the Unraid release notes to get maximum stability when using macvlan. Any reason for this?