The indication is that the password is incorrect or the account setup is incorrect. Thunderbird is just acting as the messenger between you and the email host.

This was a working existing Thunderbird e-mail account that I was using on a Dell PC running OS Windows 10 when it stopped. At that time , it displayed my inbox, files and other directories just as they where, the last time I used it. It was just a few or several seconds after I opened this Window that the pop up message appeared. Knowing that Thunderbird, as you described, acts as a messenger service, I checked for problems with the server. I checked sever response by trying my other programs on this PC and others that I have. I found Windows 7 Outlook and Windows 10 Mail where not responding. Then I tried using Webmail over the Internet and it showed failre to connect to server. I waited for Webmail to start working. Then I found Windows 10 Mail was working on this PC, but not Thunderbird. I installed Thunderbird on my Windows 7 and it works fine. I tried to uninstall and reinstall Thunderbird on my Window 10 machine. When it is re installed it does open as a new program. it open s just as it was before I uninstalled it. I am still looking to wipe out all traces of Thunderbird from my Windows 10 PC, so I will be able to make e a new, clean, and fresh installation. So far I have not been able to this.