Thanks for the quick reply. No I'm not connecting through a VPN or an incognito browser page.
Interestingly, if I do try connecting through an incognito browser window, the Web Safe settings page loads fine. But, if I then make the required change to the settings and click "Apply", I then get the same "Oops, something's broken" page. Repeating the process to get back to the Web Safe settings page shows that the settings haven't actually been changed.
So, some progress, but I still can't change the Web Safe settings.