The error is
We can't sign you in to yahoo! Messenger. There might be a problem with your Internet connection. Make sure your computer is connected to the Internet. Click Troubleshoot to begin a series of tests to try to diagnose the connection problem.
The thing is, the internet connection is totally fine, I tried most of the fixes like seeing if it's blocked in firewall or changing settings in that "messenger" box when you get to sign in or re-connecting my internet via unplugging the cable but whatever I do it doesn't work.
I am certain the username and password are typed in perfectly and it also gets even weirder: I had this problem initially with an ID named "notthationic" but, due to not being able to use it, I made a new ID and got the EXACT same problem. Keep in mind that yahoo mail is working when I sign in on both ids/mails but the messenger works on neither.