Interesting. In my case, the issue persisted even in Incognito mode, which is one of the reasons I ruled out browser extensions and cached browser data early on.
I tested multiple browsers, disabled extensions, and used Incognito mode, but the affected websites still would not load properly. The problem only went away after I disabled HTTPS Scanning under Safe Web.
You may want to check whether your Open University site starts working when HTTPS Scanning is disabled. If it does, that would suggest we’re running into the same underlying issue.