Running latest version of Thunderbird on Windows 10, one account installed (the gmail in question) and has been working reliably for over a year, all settings are still correct and without changes, there has been no recent update/restart between the last email successfully sent and the currently persisting error message.

When trying to connect to server, Thunderbird opens a second window for gmail authentification, first requesting that cookies be accepted, then failing to accept authentification: correct email is entered -> next -> correct password -> next -> back to start without authentification