I've been having a problem too. Looks like there was a recent update, but the CSS or whatever formatting tool was not installed correctly so the site isn't rendering correctly (see attached picture for what the site looks like for me presently). I can log in, but I cannot get the daily check-in popup to work, nor can I get to any of the activity pages to render either.
I've tried accessing the site from my phone, a tablet and 2 different laptops, and a desktop - from 3 different US states (ISPs) - so, no, it's not just a browser issue.
I recalled this happened once before a few years back and I had to wait for the next update of whatever broke before it fixed itself. I don't know if it's the site itself that's broken or something that broke where the site is being hosted. The site was working for me 10/23, but then I didn't try logging in again until 10/27. By 10/27 I was having this issue so I assume the update was made somewhere between those dates.
My plan was to just wait it out like last time and hope it fixed itself sooner than later.