nginx: [emerg] host not found in upstream with local IPs - nginx

Question

I've read similar questions with same error, but nothing matches my problem, because my upstream servers have local IPs.
The server is a proxmox machine with some different vms.
One is for nginx reverse gateway/proxy, the other are vms with several docker containers.
I want to setup a fallback (backup) for one container.
The config of the nginx reverse gateway/proxy containing these machines is:
server {
listen 80;
server_name my-web.page;
return 301 http://www.my-web.page$request_uri;
}
server {
listen 80;
listen [::]:80;
server_name www.my-web.page;
location / {
return 301 https://www.my-web.page$request_uri;
}
}
server {
listen 443 ssl;
server_name my-web.page;
return 301 https://www.my-web.page$request_uri;
ssl_certificate /etc/ssl/my/my-web.page.chained.crt;
ssl_certificate_key /etc/ssl/my/my-web.page.key.pem;
}
upstream backend {
server 192.168.200.210:8030 max_fails=1 fail_timeout=600s;
server 192.168.200.211:8031 backup;
}
server {
listen 443 ssl;
ssl_certificate /etc/ssl/my/my-web.page.chained.crt;
ssl_certificate_key /etc/ssl/my/my-web.page.key.pem;
server_name www.my-web.page;
location ~ ^/$ {
# rewrite only the root page, other urls see next rule
return 301 https://www.my-web-page-microsite.de/;
}
location / {
resolver 127.0.0.1 valid=30s;
# pass to backend-client, failover to second container for the next 5 minutes
proxy_pass http://backend;
proxy_set_header Host $http_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-Server-Address $server_addr;
proxy_ssl_verify off;
proxy_http_version 1.1;
proxy_set_header Connection "";
}
}
If something is wrong with my backend-client-servers, nginx won't start.
Isn't there a possibilty to override the check on starting/restarting nginx?