So this is in no way the best solution, but it seems like the issue in Internet Explorer occurs when the browser is trying to access just the URL: example.com/sub-installation
When I simply add a slash to the end of it, the browser will try to access the front page on example.com/sub-installation and not try to access a sub-page to example.com.
It doesn’t really solve the issue for good, but it works for now….