Redmine

Several places in Redmine (including the generation of redirect URLs, some API methods, and other places) leverage the given Host header in the current request to generate full URLs.

Thus, you have to pass on the original Host header to Redmine as well as an indication whether the original request to nginx was made over https. For that, you need to include the following settings into your proxy block:

proxy_set_header Host $host;
proxy_set_header X-Forwarded-Proto $scheme;

Please refer to the documentation of the proxy_set_header rule in nginx for more details and possible adaptations depending on your setup.