What does it actually do? When it's on, I get a redirecting error when trying to connect to my nextcloud at cloud.example.com, but with it off, it seems like I can't connect to NC using http anyway? Am I misunderstanding what it does?
If my understanding of how "force SSL" works for most proxies, it just simply issues a HTTP 300 redirect message for all http traffic coming in on port 80. It then sends everything to port 443 https.
Do you get a 502 when you try to connect with the force SSL turned off? It might me less of an issue with SSL and more that your proxy is not pointing to the right host / port of your nextcloud server.
When I connect with it off, I am able to connect. If I use https://cloud... It connects, when I use http://cloud... The address bar changes to https and connects like normal.
It just feels weird using it with force SSL off, as if even though I'm always connecting with https I'm still leaking passwords
“Force SSL” forces your browser to redirect http://example.com to https://example.com, it doesn’t “force” SSL into the connection. As long as you’re connecting through HTTPS, the traffic is encrypted.
If your browser redirects you from http to https after you turn off Force SSL, there could be two things going on, both related to caching:
You turned on HSTS previously and that’s still cached in your browser.
The http->https redirect is still cached.
You can check both of these by clearing your browser cache for example.com (or in general).
Usually it's due to the webapp not knowing that the request is actually already served under https by the reverse proxy, it think the request is still served over http so it issues a redirect to https, which leads to a redirect loop.
reverse proxy pass the requests to upstream server, but using http instead. It also add HTTP_X_FORWARDED_PROTO header to let the upstream server know that even though the request is over http, the end user actually served over https so it should assume the connection as https
the upstream server return a response, it take into accounts that the request is already done via https (e.g. generating links with https:// prefix, etc).
I think this chain is broken somewhere in your setup and cause nextcloud to not receive the HTTP_X_FORWARDED_PROTO header from nginx proxy.
I just remember nextcloud has trusted_proxies option located in config/config.php file. Not configuring it (or configured it wrong) can cause nextcloud to not trust the HTTP_X_FORWARDED_PROTO header it received (it assumed to be spoofed).