I have been using Gmail accounts inside Thunderbird for some time and every now and then I received those annonying error messages stating that the username or password was invalid. This was somehow a conundrum as the passwords were saved within the Thunderbird profile and had not changed.
Looking up the issue today I came across both the problem, the trigger, and the solution. I realised that Gmail does not let you check for mailbox changes more often than every 10 minutes. Thunderbird was set to check for new mail every 6 minutes which seems to have triggered the account to be locked out. First when the Captcha had been passed the email account was again available via IMAP.