Howdy @ssrlive, welcome to the Caddy community.
I can’t say for sure because you haven’t actually included the request details of the bad response, but my hunch is that possibly you want /mypathWiWL/* instead of /mypathWiWL/ for your reverse_proxy.
Remember that path matchers in Caddy are LITERAL PATHS and require globs if you want them to match a prefix. This is different from nginx where location directives are automatically path prefixes, or optionally regex.
Remember also that Caddy returns 200 OK for any response where nothing was configured to provide a different response.