What is the exact error message?

This is the window that pops up -

Accounts Help Accounts

   Troubleshoot issues with Google Accounts

Enable or disable cookies

If you’re trying to use your Google Account and see a message saying your browser's cookie functionality is turned off, it’s likely your browser's cookies are disabled. You’ll need to turn cookies on to use your Google Account. How to turn on cookies Google Chrome

   Select the Chrome menu icon Menu.
   Select Settings.
   Near the bottom of the page, select Show advanced settings.
   In the "Privacy" section, select Content settings.
   Select Allow local data to be set (recommended).
   Select Done.

Learn more about managing your cookies settings in Google Chrome, like how to block third-party cookies. Other browsers

Please see your browser's help resources. How to turn off cookies Google Chrome

   Select the Chrome menu icon Menu.
   Select Settings.
   Near the bottom of the page, select Show advanced settings.
   In the "Privacy" section, select Content settings.
   Select Block sites from setting any data.
   Select Done.

Other browsers

Please see your browser's help resources. How enabling cookies helps you

Have you tried to allow cookies for https://accounts.google.com in Thunderbird?

I just checked and lo and behold cookies was off. I don't know how that happened, but I turned it on and it worked. Thank you!

Good. Can you then mark the thread as 'Solved' please? Thank you.