Can't Create Configuratio for mail.com

For no obvious reason, Thunderbird started complaining about my existing configuration to mail.com. I tried changing passwords, I tried rebooting the computer, but nothing would work. I kept getting the "invalid password" popup when trying to connect. I've double-checked that mail.com is still working (I can access all my email from the web client no problem). So, eventually, I deleted the account from Thunderbird altogether and now I'm trying to recreate the configuration. Thunderbird recognizes mail.com and correctly deduces the server configuration settings, but it will not connect. When I try to add the account I get this message: "Authentication failed. Either the entered credentials are incorrect or a separate username is required for logging in. This username is usually your Windows domain login with or without the domain" Why on earth would a "Windows domain login" be needed for a web-based email location?? When I had the configuration set up before, all I had to supply was my email address and password. Something in Thunderbird has changed and I can't figure this out. Meanwhile, I have about 10 other email accounts set up on Thunderbird and they all seem to be working properly. Would sure be nice to find a solution so I can get my mail.com account back on Thunderbird. Thanks for any help you can provide.