The "Network error" in the top left is a JS result. Also, since you linked to the source view, you'll see that there actually isn't anything in the body but the script being loaded, whereas "Full version" shows the user interface was correctly initialised.

Disable JavaScript in your browser when viewing the cached page. All you will see is an empty page.

Now I'm feeling a bit stupid, will slowly walk away and hope I can get by with "it's Jan 1st and I didn't get much sleep" as an excuse...

First, thank you for taking the time to stick your neck out a bit and share an example.

Second, thank you for responding to someone pointing out that you were wrong without putting yourself on the defensive.

I see this entire sub-thread as a positive; glad we all could learn along with you!