I figured it out.by accident. I tried removing the "http://" prefix in the url, to no avail. I did get a 404 error. And in the access log of ProxMox I also had those 301 errors. I accidentally prefixed the url of the load balancers server with "https://" and it suddenly worked. I had missed that the service was internally also using https, with a self-winding certificate. I'm sorry that you had to go through these efforts, but thank you for your help!