My intuition based on the error log is that a good place to start is SSL. It looks like it's failing to establish the ssl link. Maybe in your nginx configuration start off by trying to connect using http on port 80, and if you can get that to work it means you've got something going on with your ssl. If it doesn't work, then you can move on to the link between nginx and lemmy. I suspect you should be able to use links or lynx to access http://localhost:21560 on the server node to see if it is accessible. (If it isn't accessible, then your next step is trying to figure out why you can't even find lemmy on the local node using a web browser)

I use apache as my reverse proxy (and I don't run lemmy, but I run a number of other services), and usually the first step is to get the service running and try to connect to it locally, and once you've successfully set it up so you can connect locally it becomes relatively easy to set up the reverse proxy, first using port 80, then on 443 (and disabling port 80).