Unsafe connection after commiting a file over the web editor


I use GitLab as a self-hosted system on my server with an Nginx reverse proxy and SSL behind it. My GitLab instance is displaying a notification about an unsafe connection after editing and commit a file over the web editor.

Nginx uses the following configuration for the GitLab subdomain:

server {
	listen 						[::]:443 ssl;
	listen 						443 ssl;
	server_name  				gitlab.server-kampert.de www.gitlab.server-kampert.de;

	client_max_body_size		0;

	access_log 					/var/log/nginx/gitlab_access.log;
	error_log 					/var/log/nginx/gitlab_error.log;

	location / {
		proxy_pass 				http://localhost:8101;
		proxy_set_header 		Host $host;
		proxy_set_header 		X-Real-IP $remote_addr;
		proxy_set_header 		X-Forwarded-For $proxy_add_x_forwarded_for;
		proxy_set_header 		X-Forwarded-Proto $scheme;
		proxy_set_header 		X-Forwarded-Protocol $scheme;
		proxy_set_header 		X-Url-Scheme $scheme;

    ssl_certificate 			/etc/letsencrypt/live/server-kampert.de/fullchain.pem; # managed by Certbot
    ssl_certificate_key 		/etc/letsencrypt/live/server-kampert.de/privkey.pem; # managed by Certbot

So what is wrong here?