Hi welcome to fixya. Hope u 'll be fine

your request is a bad or oversized cookie. (The same thing is true for request Error 400").
Simply solution is delete your "Cookies" Don't worry about erasing them;nothing bad will happen. You won't lose your internet browser "Bookmarks" or "Favorites"
As you're dumping your cookies, you should also erase all your "Temporary Internet Files" and "History". When u 'll d this your computer 'll be totally fresh in YouTube's.
After doing this restart your PC
whatever u r using browser to delete cookies, history and temporary internet files Go to Tools > Internet Options
See where is the option of "Delete Cookies and Clear History. Click on respective options to delete them

I hope this solutions will work and helpful 4 u. Best of luck
Please do rate if it is helpful
THANKS & keep using fixya