CORS Header missing in Firefox although add_header is used - nginx

Question

I have an application that consists of back end and front end. Because of restrictions with the hoster, I need to provide the back end from a different server than the front end.
My back end handles authentication and serves the content to the front end. It also sends emails to users via nodemailer. Because I am not allowed have outgoing TCP sockets on the server where the front end is hosted, this feature failed which made me relocate the back end.
Now, I have the back end running on a different server. It consists of a loopback instance listening on a certain port which gets requests proxied to it by nginx.
After a while of set up, I had the configuration working. It first failed because of a wrong CORS header, a problem that emerged because Loopback added Access-Control-Allow-Origin *;, which I also had in my nginx config. That resulted in Firefox throwing an error like CORS header does not match Origin (*, *) - which made me think that the headers where on top of each other thus negating the wildcard *.
So I removed the add_header part from my nginx configuration. I worked fine when I tested, but when I came back, Firefox threw the Error Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://{{my_nice_api}}/lang. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing). Status code: 404.. Which baffled me because I hadn't changed the set up at all.
Now, I fiddled even more but am not able to find the error. I have add_header Access-Control-Allow-Origin *; set (for testing purposes obviously), but I keep getting the error that there is no such header present. This post had me thinking that I needed to add another header in Access-Control-Allow-Credentials true;, but to no avail. Can anybody give any pointer as to what I am missing?
My nginx.conf:
user www-data;
worker_processes auto;
pid /run/nginx.pid;
include /etc/nginx/modules-enabled/*.conf;
events {
worker_connections 768;
# multi_accept on;
}
http {
##
# Basic Settings
##
sendfile on;
tcp_nopush on;
tcp_nodelay on;
keepalive_timeout 65;
types_hash_max_size 2048;
# server_tokens off;
# server_names_hash_bucket_size 64;
# server_name_in_redirect off;
include /etc/nginx/mime.types;
default_type application/octet-stream;
##
# SSL Settings
##
ssl_protocols TLSv1 TLSv1.1 TLSv1.2 TLSv1.3; # Dropping SSLv3, ref: POODLE
ssl_prefer_server_ciphers on;
#ssl_certificate /etc/nginx/certs/cert.pem;
#ssl_certificate_key /etc/nginx/certs/key.pem;
##
# Logging Settings
##
access_log /var/log/nginx/access.log;
error_log /var/log/nginx/error.log;
##
# Gzip Settings
##
gzip on;
# gzip_vary on;
# gzip_proxied any;
# gzip_comp_level 6;
# gzip_buffers 16 8k;
# gzip_http_version 1.1;
# gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;
##
# Virtual Host Configs
##
include /etc/nginx/conf.d/*.conf;
include /etc/nginx/sites-enabled/*;
}
My site.conf (mounted into sites-available):
# Virtual Host configuration for {{my_nice_api}}
#
server {
listen 80;
listen [::]:80;
server_name {{my_nice_api}};
return 301 https://{{my_nice_api}}$uri;
#location / {
# rewrite ^ https://{{my_nice_api}}$request_uri permanent;
#}
}
server {
listen 443 ssl http2;
listen [::]:443 ssl http2;
ssl_certificate /etc/nginx/certs/cert.pem;
ssl_certificate_key /etc/nginx/certs/key.pem;
#server_name {{my_nice_api}};
location / {
proxy_pass http://localhost:3001/api/;
#proxy_pass_request_headers on;
#proxy_http_version 1.1;
#proxy_cache_bypass $http_upgrade;
#proxy_set_header Upgrade $http_upgrade;
#proxy_set_header Connection "upgrade";
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-Host $host;
#proxy_set_header X-Forwarded-Port $server_port;
add_header Access-Control-Allow-Origin *;
add_header Access-Control-Allow-Credentials true;
#add_header X-Frame-Options SAMEORIGIN;
}
}