I am unable to disable the plug-ins (via the WP Dashboard anyway). When I attempt to do so, that link opens up the website, not the plug-in page. The same thing happens when I try to go to the Theme page from within the Dashboard. It just seems like the Dashboard is the issue for some reason, but I certainly could be mistaken.
If I revert back to that other them, what will happen to my existing content & settings when I go back to my current theme (which is ThemeCo X)? Do I need to backup my current site?
Yes, I have tried this on a completely different browser and cleared the cache with the same results.