If you are unable to log in to your Yahoo account, then you should clear all cache of the browsers. Now, restart your system and use a different browser to check if issues are resolved. Apart from this, check your user’s credentials as well as internet connectivity. If internet connectivity is poor, then it takes more time to load a new page and shows an error message. For further, speak to a Yahoo representative.
https://www.supporttechhelp.com/yahoo-customer-service/