Vaadin23 with NGINX and websockets

Question

I run NGINX in front of Vaadin 23 Spring Boot application. From time to time the web application becomes unresponsive. In the browser console, I see the following 501 error:

GET wss://example.com/?v-r=push&v-uiId=6&v-pushId=bc1ba5ed-b57c-48eb-ae1d-beba3104d29e&X-Atmosphere-tracking-id=0&X-Atmosphere-Framework=3.1.2-javascript&X-Atmosphere-Transport=websocket&X-Atmosphere-TrackMessageSize=true&Content-Type=application/json; charset=UTF-8&X-atmo-protocol=true

What might be causing this?

UPDATED

NGINX config:

server {

    root /var/www/html/domain.com;

    index index.html index.htm index.nginx-debian.html;

    server_name domain.com;

    sendfile                   on;
    tcp_nodelay                on;
    tcp_nopush                 on;
  
    etag                       off;
    if_modified_since          off;
  
#   proxy_buffering            off;

    proxy_buffer_size          128k;
    proxy_buffers              4 256k;
    proxy_busy_buffers_size    256k;

    proxy_cache                off;
    proxy_cache_convert_head   off;
    proxy_max_temp_file_size   0;
    client_max_body_size       0;
  
    proxy_http_version         1.1;
    proxy_pass_request_headers on;
    proxy_pass_request_body    on;
  
    proxy_read_timeout         1m;
    proxy_connect_timeout      1m;
    reset_timedout_connection  on;
  
    proxy_redirect             off;
    resolver                   77.88.8.8 77.88.8.1 8.8.8.8 8.8.4.4 valid=1d;
  
    gzip                       off;
    gzip_proxied               off;

    location / {

        include /etc/nginx/snippets/cors.conf;
        proxy_pass http://127.0.0.1:8080;
            proxy_redirect http://127.0.0.1:8080 /;
            proxy_set_header Host $host;
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_set_header X-Forwarded-Proto $scheme;
            proxy_set_header X-Real-IP $remote_addr; 
    }

     location /api {
            proxy_pass http://127.0.0.1:8080;
            proxy_redirect http://127.0.0.1:8080 /api;
            proxy_set_header Host $host;
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_set_header X-Forwarded-Proto $scheme;
            proxy_set_header X-Real-IP $remote_addr;
        }
}

UPDATED

I added the following:

    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection "Upgrade";
    proxy_read_timeout 3600;
    proxy_connect_timeout 600;

to the location / section but still ran into the following issue: Vaadin23 Uncaught (in promise) Error: Client is resynchronizing