Are you getting an error message when you sign in? I'd agree with Fiat Lux - Asia about using a private browser. Here are the keyboard shortcuts:
Google Chrome: Ctrl + Shift + N
Firefox and MS Edge: Ctrl + Shift + P
Safari: Command + Shift+ N
If you can sign in, you can go back to your regular browser and clear its cache. If not, please try a different browser to see if the behavior only occurs on the one you're using or not.