I just had this same problem happen and it's fully reproducible. It looks like the issue is with the redirect after logging in. The original article is getting stripped from the URL and just left with this:
If you use the back button in the browser (or re-click the original link), the article will load fine.
Maybe @Heidi can take a look at this...
Hi all, I have just checked and this issue is still on the radar for our web team. They are currently looking at fixing it next month. I'll post to this thread when I have a further update. Thanks, Heidi