SyntaxError: JSON.parse: unexpected end of data at line 1 column 1 of the JSON data
This is an error you are getting in your web browser with lemmy-ui? Do you know how to use the console of Firefox or Chrome to look at what API call that error is coming from?
Caddy as reverse proxy, but I left nginx
Could be related to your problem... as lemmy-ui doesn't seem to be talking to the API to get JSON responses. I suggest looking at your proxy logs at the instant you press submit on the form.