You'll likely have to add the webroot to the "to" directive so it will find the index file if it's installed in a subdirectory.
The confusing thing is that both Urbackup and SABnzbd+ are installed at the root--the "to" addresses I gave in my example are the addresses I use to access them. In the case of Urbackup, no path is ever added as far as I can see. With SAB, a path is added if I go to pages other than the main page (/config/general, /config/folders, etc.), but not on the main page.
I see enough examples of configuring Apache or nginx to reverse proxy for SABnzbd+ that I'm sure it can be done, and none of them mention any changes to SAB's configuration, so the behavior I'm seeing is somewhat puzzling.
I did get some logging going on caddy, and here's what I'm seeing:
Code:
192.168.1.241 - [20/May/2016:21:16:03 -0400] "GET /urbackup HTTP/1.1" 404 22
192.168.1.241 - [20/May/2016:21:16:52 -0400] "GET /urbackup HTTP/1.1" 404 13
192.168.1.241 - [20/May/2016:21:18:51 -0400] "GET /urbackup/ HTTP/1.1" 404 22
192.168.1.241 - [20/May/2016:21:23:00 -0400] "GET /nzb/ HTTP/1.1" 303 98
192.168.1.241 - [20/May/2016:21:23:32 -0400] "GET /nzb/ HTTP/1.1" 303 98
The "404 13" line results in "404 Not Found" in the browser, while the "404 22" gives "Sorry. File not found."