That’s strange – if you have many browser tabs open, try to close most of them except the one you’re using for YouTube. Also, if you haven’t already, refresh your browser and clear cache & cookies, then try signing in again. Let us know if that helps!
Hmm, that's odd. Does this happen across devices? Try to do the basic troubleshooting steps. This usually does the trick: https://yt.be/help/sWdH . Keep us posted!