System.CalloutException: Unexpected end of file from server

The Salesforce requests(Callouts) receiving server may have a firewall and blocking the Salesforce IP addresses. So, whitelist Salesforce IPs. Salesforce IP Addresses and Domains to Allow is below
https://help.salesforce.com/articleView?id=000321501&type=1&mode=1

Sometimes, it usually means the server that you are calling out to isn’t responding correctly. The server takes very long to process the data which results in the servers connection to timeout before it could send a response.