Well, it turns out the loading error was connected to the logo not being displayed.
Inspecting the page source, I saw that the logo was pointing to a location on the development site.
I just reloaded the logo in the header and all was well.
I suspect the reason it was only an issue on my iMac and not my PC, iPad and iPhone is because I had used all those devices during testing and the image was in the cache.
This issue can be closed.
