by Ken Task.
Strange!
What's your version of PHP and does your server load php-soap?
The directory the error points to contains only a webservice_soap.php php file and inspecting it appears to strings related to soap:
$string['pluginname'] = 'SOAP protocol';
$string['privacy:metadata'] = 'The SOAP protocol plugin does not store any pers$
$string['soap:use'] = 'Use SOAP protocol';
And, your version of moodle could stand an update as 4.1 is now 4.1.13+ and is only getting security fixes now ... no code fixes.
'SoS', Ken