I have caddy working beautifully for my site, reverse proxying HTTPS for all my services. It’s amazing how nicely it works.
However, I have one device (my kobo) that doesn’t seem to like the https access for the link to the calibre-web api. I have calibre-web configured on its own domain, for secure web access.
Enable the debug global option. What do you see in your logs when you try to use your Kobo?
How old is your Kobo? It might be using an outdated (insecure) TLS version if it’s too old.
It is possible to serve it over HTTP, but I strongly recommend figuring out why it’s not working before taking that approach. Turning off security should always be the last resort.