It seems this is still going on as of May 14th, but I don't believe it's any kind of "server" issue. I use to work for Apple Inc. (not store) and you'll can get a different story every time - depending on who you talk with. ;-)
I have several Macs but it's been a while since I've used the one I'm on now. Similar model, but OLDER.
[27"/ 3.4ghz / i7/ 32gb ram - macOS High Sierra ]
(BTW, Boy... I don't know why you're not running a newer macOS on yours though? You can. I would!! :-)
Rather than any kind of "server" issue messing with the App Store UI - I suspect hardware and/or software. All my other Macs are not doing this, older and newer models. Plus... all the other areas of the Mac App Store are just fine for me. I think it's going to take a little more sleuthing and trouble shooting.
My weird "boxed" update buttons and check boxes are working though. I was able just able to download one of my updates. Back to the drawing board.