Usually when the website doesn’t work on XP, it means that you are using SNI (one IP has several https sites on it), however, that’s not your case here…
What browser do you use to access the site? (Chrome seems to be fine, but IE does have some issues, might be due to FS)
I’m using IE, my real problem is that the site is unreachable from my program written in c# if it is started on Windows XP
PS The WebClient used in the code is precisely InternetExplorer
This isn't a problem of the certificate, it's a problem of your webserver-configuration.
Very old (XP): One ip-address -> one domain and one certificate
Newer (Server Name Indication - SNI): One ip-address + a lot of domains + a lot of certificates -> Client sends the server name, server answers with the correct configuration.
So you need to change your configuration that your standard entry (default) has the correct certificate.
No, this is not the problem. The problem is, that the client calls the ip-address and don't send the hostname / domain name (there is no place to send the domainname without SNI).
So the server has to select the right certificate. That is impossible if there are a lot of domains.
You can test it with
So your domain with XP-support has to be the default domain.