I'm guessing that you are trying to connect from Nginx to your Nextcloud appliance via http?! I say that as by default the appliance is configured to redirect plain http to https. So probably the best way to resolve the issue is to connect via https instead, then there shouldn't be any redirect.

Assuming that you are using valid self-signed certificates, then I'd recommend regenerating all your desired certs on your Nextcloud appliance and copy the ca-cert to the Nginx proxy to use. Then add this to your Nginx conf:

proxy_ssl_verify on;
proxy_ssl_trusted_certificate /path/to/your_selfsigned_ca_cert.pem

Alternatively, if you don't want to worry about locking it down that much, then you could just disable verifying the https cert:

proxy_ssl_verify off;

Alternatively, you could disable the redirect in the Nextcloud appliance and use vanilla http. (i.e. the Rewrite components within the port 80 section of the /etc/apache2/sites-available/nextcloud.conf file).