nginx: proxy_pass subdirectories to other servers - ssl

Question

Im using nginx as Web Server and Reverse Proxy with SSL enabled.
The Web Server serves an WoltLab Suite Forum 5.0.0 (formerly known as Burning Board) and proxied some subdomains to different hosts like a NodeJS backend, an Tomcat backend and many other different services.
This worked great so far, but now i have the problem that i can no longer use subdomains to accomplish this.
Please don't ask why, please don't.
Now, that i can no longer use subdomains, Im trying to get it working with sub directories.
An example:
I had xyz.example.com point to my nginx server at 12.13.14.15.
nginx proxied all requests to xyz.example.com to 10.20.30.40:1234
Now i want nginx to proxy all requests to example.com/xyz/ to 10.20.30.40:1234
I got this working with Apache Archiva as backend service, but all other services like my NodeJS backend are refusing to work correctly with my current configuration.
It sends me to the BurningBoard wich shows me their Page-Not-Found page.
example.com/xyz/admin/index.php becomes to example.com/admin/index.php wich wont work of course.
The directory that proxies to Archiva has the exact same configuration, just other directory names of course.
The Archiva URL looks like this after i call it from Web:
example.com/repo/ becomes example.com/repo/#welcome and shows me Archivas Welcome Page.
This is exactly what i want for my other services too.
Here are my current configuration files for nginx (sensitive data replaced with X):
<=== sites-available/default ===>
server {
listen 80 default_server;
listen [::]:80 default_server;
server_name XXXX.XX www.XXXX.XX;
return 301 https://$server_name$request_uri;
}
server {
listen 443 ssl default_server;
listen [::]:443 ssl default_server;
include snippets/ssl-XXXXX.XX.conf;
include snippets/ssl-params.conf;
root /var/www/html;
include /etc/nginx/snippets/proxies.conf;
# the last try_files argument is for SEO friendly URLs for the WSF 5.0
location / {
index index.php;
try_files $uri $uri/ /index.php?$uri&$args;
}
location ~ \.php$ {
include snippets/fastcgi-php.conf;
fastcgi_pass unix:/var/run/php/php7.0-fpm.sock;
}
# Lets Encrypt automation
location ~ /.well-known {
allow all;
}
location ~ /\.ht {
deny all;
}
}
<=== snippets/proxies.conf ===>
# Apache Archiva
location /repo {
rewrite ^/repo(/.*)$ $1 break;
proxy_pass http://XXXXXXXX:XXXXX;
return 404;
}
# Git Solution
location /git {
rewrite ^/git(/.*)$ $1 break;
proxy_pass http://XXXXXXXX:XXXXX;
return 404;
}
# Filehosting
location /cloud {
rewrite ^/cloud(/.*)$ $1 break;
proxy_pass http://XXXXXXXX:XXXXX;
return 404;
}
# NodeJS
location /webinterface {
rewrite ^/webinterface(/.*)$ $1 break;
proxy_pass https://XXXXXXXX:XXXXX;
include /etc/nginx/snippets/websocket-magic.conf;
return 404;
}
Any ideas how to solve this problem?
Also, please tell me if you need more informations like nginx' version or the like.