The webservices plugin is now loaded in the server so the process of enabling logs has changed. You need to edit the ARSystem\conf\logback_server.xml file as detailed in the docs here – AR System and web services introduction – BMC Remedy Action Request System 9.1

To enable logging for web services

1. Open the <ARInstallationFolder>\conf\logback_server.xml file for editing.

2. Update the file with the following lines:

    <logger name="com.bmc.arsys.ws.client.WebServiceSoapClientImpl"  

               level="debug" additivity="false">

               <appender-ref ref="ServerLog" />

    </logger>

3. Restart the AR System server or use the Reload Log Conf File setting on the Server Information form, Log Files tab.

Note: If you have provided the appender-ref as ServerLog, the logs for web services will be generated in arerror.log file.