Non-functional buttons would probably be a broken or missing script problem, but there could also be stylesheets missing. eBay might have jockeyed some URLS around and both scripts and stylesheets are not loading in some cases.
I don't have a store so can't look.
You might be able to see if something is failing to load using the network tab of the web developer tools in Chrome or Firefox (CTRL-SHIFT-Q in Firefox) If you open that tool and monitor a load of that store management page by reloading it via F5, it should show you pages that return a 404 not found status on the left. Look for things like
https://ir.ebaystatic.com/rs/v/10341xh50yz21mhhydueu4m5wad.js or files that end in .css failing to load. 304 status indicates a cached file - that would be a good time to do a full page reload via CTRL-F5 to force a reload from the server, and see if anything changes on the page. That exercise might reveal why the page is failing.
This is the type of problem that clearing cache (and cookies for good measure) can sometimes fix in cases where ebay swapped out some scripts or stylesheets, and your browser is still pulling the stale, cached versions. (My goto "try an older browser" is probably not a workaround for this type of thing.)
Another possibility would be to try a Seller Hub path instead of the My eBay path to that functionality if it exists. Start here and see if there are store management features you can access from the menu bar at top, or the shortcuts, or seller tools panels:
https://www.ebay.com/sh/ovw
