Aaaaaaa HAAAAA! So that’s the problem. You nailed it; I have duplicati as a subfolder of the root.
I run both duplicat and Caddy on the same local machine, and set the Caddy server to a sub domain like caddy.mysite.com:1234.
Then try to set all reverse proxies through that. In most cases, the sites that run on their own server + port I can just do the usual proxy /whatever 127.0.0.1:4321, and set the baseurl of /whatever in it’s settings, but duplicati doesn’t like that.
I guess I’ll just create another subdomain in the caddyfile just for duplicati, and maybe add that subdomain to my hosts file, or something. I’m only wanting to use duplicati locally, to backup local machines.
Side note, while researching the issue I had come across another post where someone suggested using the Filter plugin to append the baseurl to all pages, to ‘force’ it when the web app itself doesn’t already have it built-in; i cant find the original post, but doing a search just now came up with an even better one: https://caddy.community/t/solved-add-a-base-tag-when-reverse-proxying-with-a-subirectory/4682/3
I wonder if this would be just as good? I guess I’ll just have to give it a try. 
Anyway, thanks a million for taking the time to install it and figure it out, it’s genuinely appreciated!