Https://www.google.com and search words yahoo bad request gives a number of people with error, particularly with ATT.
Here is an example:
Quoting from https://forums.att.com/t5/AT-T-Internet-Email-Security/My-att-yahoo-email-comes-back-with-a-Sorry-Bad-Request-How-do-I/td-p/5227973/page/2 : Aug 30, 2017 6:57 PM Re: My att yahoo email comes back with a Sorry Bad Request. How do I fix it? I had the same problem but I think I may have stumbled upon how to fix the issue. I had the same trouble with my 5 sub accounts and now I have them all working. on a desktop computer go to www.att.net and log in with your primary email account. Once logged in click on the email icon to go to your inbox. Once your inbox opens if you look to the top right you will see the icons for HOME then YOUR NAME then the GEAR icon. Hover the mouse over the icon where your name or whatever your account is called a menu will pop at the bottom it will say Add or Manage accounts click on that and once that page loads click the blue add account icon and it will ask you to log in use the sub account user name and password and it will take you through the account set up for the sub account click next and it will activate the sub account. I went through this process tonight and got all 5 of my sub accounts working. Before doing this procedure as described even tonight I was still getting the Bad Request error message. I hope this works for you. englishman"
Answers & Comments
Https://www.google.com and search words yahoo bad request gives a number of people with error, particularly with ATT.
Here is an example:
Quoting from https://forums.att.com/t5/AT-T-Internet-Email-Security/My-att-yahoo-email-comes-back-with-a-Sorry-Bad-Request-How-do-I/td-p/5227973/page/2
:
Aug 30, 2017 6:57 PM
Re: My att yahoo email comes back with a Sorry Bad Request. How do I fix it? I had the same problem but I think I may have stumbled upon how to fix the issue. I had the same trouble with my 5 sub accounts and now I have them all working.
on a desktop computer go to www.att.net and log in with your primary email account. Once logged in click on the email icon to go to your inbox. Once your inbox opens if you look to the top right you will see the icons for HOME then YOUR NAME then the GEAR icon. Hover the mouse over the icon where your name or whatever your account is called a menu will pop at the bottom it will say Add or Manage accounts click on that and once that page loads click the blue add account icon and it will ask you to log in use the sub account user name and password and it will take you through the account set up for the sub account click next and it will activate the sub account. I went through this process tonight and got all 5 of my sub accounts working. Before doing this procedure as described even tonight I was still getting the Bad Request error message.
I hope this works for you.
englishman"