I can open my reputation and the Stack Exchange dropdown, but when I go to hover over my Inbox, it loads for a fraction of a second and then disappears.
I had 3 notifications and when I clicked on the inbox symbol, they disappeared, so it assumes I've read them but I have not.
Here is a GIF of the problem:
I have managed to reproduce this in all of the browsers I have installed (Chrome, Edge, IE, Firefox and Opera).
I get an error in the console: