Hi Mari,
We were aware this worked with other browsers and versions of IE but unfortunately, as stated in my original post, upgrading wasn't an option. Large institutes have other factors to consider when configuring/upgrading a default internet browser.
Turnitin supplied this information in August regarding the issue: A quality assured, pre-release, plugin will be available via GitHub soon after it goes through QA testing. The pre-release plugin will resolve the issue with the use of Internet Explorer 9.