Authentication Required Window constantly pops up

I have a ton of email accounts in Thunderbird on Mac. I recently added a new account, and it constantly pops up the Authentication Required - Mozilla Thunderbird window. Below that it says: https://OURURL is requesting your username and password. The username and password are both stored in the passwords, and I don't get the password needed message, and can email and receive in between the annoying pop up. I have this same issue on a new computer we just set up with other email addresses too, so I think it has to do with some automatic setting when adding a new account that happened in the most recent release. I do not get the authentication message for any of my older accounts. I tried all the things I could find online - deleted and re-added account, deleted the password files in the profile folder, reset the startup cache, etc. I searched the support base, and can't find anything We host our own email through cPanel. All the server settings are correct, or we wouldn't be able to use the email address at all.