Routing to JATOS app via Nginx reverse-proxy

I have a reverse-proxy setup to route appropriate requests to my (docker-composed) JATOS server. The proxy is there to allow my teammates to run different online-experiment applications on the same machine. I want to configure the proxy in such a way that users land on a generic welcome page when visiting our server (e.g., www.example.com) and land on the JATOS admin page only if they specify the location appropriately (e.g., www.example.com/jatos). However, the "default" Nginx configs provided in the documentation redirect all traffic to the JATOS server:

...

server {
    listen                      443 ssl;
    server_name                 memo.hib.uni-tuebingen.de;
    ssl_certificate             /etc/nginx/certs/memo-hib_cert.pem;
    ssl_certificate_key         /etc/nginx/certs/memo-hib_key_np.pem;
    ssl_protocols               TLSv1 TLSv1.1 TLSv1.2 TLSv1.3;
    ssl_prefer_server_ciphers   on;

    location /app1 {
        proxy_pass http://app1/;
    }

    location /app2 {
        proxy_pass http://app2/;
    }

    # JATOS traffic
    location / {
        proxy_pass              http://jatos-backend;
        proxy_connect_timeout   300;
        proxy_send_timeout      300;
        proxy_read_timeout      300;
    }

    # Websocket location (JATOS' group and batch channel and the test page)
    location ~ "^/(jatos/testWebSocket|publix/[a-z0-9-]+/(group/join|batch/open))" {
        proxy_pass              http://jatos-backend;
        proxy_http_version      1.1;
        proxy_set_header        Upgrade $http_upgrade;
        proxy_set_header        Connection $connection_upgrade;
        proxy_connect_timeout   7d; # keep open for 7 days even without any transmission
        proxy_send_timeout      7d;
        proxy_read_timeout      7d;
    }
}

When I change location / to location /jatos, and add a location for the welcome page, Nginx successfully routes requests where they are supposed to go, but now the stylesheets (and probably more stuff) seems to be missing from JATOS:

and:

In the proxy container logs, I get messages like:

nginx.1     | 2022/08/18 16:00:31 [error] 242#242: *589 open() "/var/www/html/assets/lib/bootstrap/css/bootstrap.min.css" failed (2: No such file or directory), client: 134.2.86.39, server: memo.hib.uni-tuebingen.de, request: "GET /assets/lib/bootstrap/css/bootstrap.min.css HTTP/1.1", host: "memo.hib.uni-tuebingen.de", referrer: "https://memo.hib.uni-tuebingen.de/jatos/login"

It looks like Nginx routes the request for the stylesheets to var/www/html/assets/lib/bootstrap... which are not there. Any idea on how to fix this?