Soap error - failed to load external entity

I have a PHP-script which uses SoapClient that connects to an API and runs methods. It works on all the different servers I have run it on previously but when I tried it on a server I get the error:

SOAP-ERROR: Parsing WSDL: Couldn't load from 'https://example.com?WSDL': failed to load external entity 'https://example.com?WSDL'

I have my thoughts that there is trouble with the server connecting to a https-domain. What server-library is Soap using in the background? Curl?

Thanks in advance!