the update screenshot preview (loupe icon?) button on this page doesn't seem to provide a screenshot. it refreshes the page and shows me the correct rack in small format.

It refreshes it in the background.
That is a good sign. If the rack is correctly displayed in small format, that means the screenshot generation works.
Can you please check this:
In Command Center click the first Icon of a thumbnail: Zoom In.
It opens the (old) screenshot as a jpg in the browser.
The URL looks like this:
https://cdn.modulargrid.net/img/racks/modulargrid_334231.jpg

Now change that url by adding ?ts=12345 [12345 can be a random number] like this and hit enter.
e.g.
https://cdn.modulargrid.net/img/racks/modulargrid_334231.jpg?ts=12345
Does this do any good?

Beep, Bopp, Bleep: info@modulargrid.net