"Third-party sign-in" may have been disabled. This can happen if you closed the Google sign in. Google calls this a feature...!
Go to chrome://settings/
Click on "Privacy and Security" (which takes you to chrome://settings/content/all)
Click on mail.google.com
Scroll down to "Third-party sign-in"
Make sure this is set to "Allow (default)" (Block disables Google sign in).
If you can't or don't sign in the extension will not load.
Then reload Gmail. You could also try
deleting your cache and then re-loading Gmail.
G Suite (now Google Workspace) users: Your Gsuite/Google Workspace administrator could be not allowing extensions, see:
https://support.google.com/chrome/a/answer/6306504?hl=en&ref_topic=6178561
If you're signed into multiple Gmail accounts you will see a prompt asking you to confirm which account you want to sign into when the extension loads, if you select a different account than the current gmail you are using you will get an alert that you are trying to sign in using the wrong email address.
Select the correct email and you will be signed in. If you get the warning prompt again despite selecting the correct email, sign out of all Gmail accounts and then clear your browsers cache.
Before clearing your cache ensure any key pairs and public keys are downloaded first. In this Scenario we recommend clearing
Cookies and other site data:
How-to-delete-Chrome-Browser-Cache.
We store your key pairs (Private & Public keys) in the browsers Local Storage. In theory Chrome Local storage is meant to persist forever, however
we have observed that occasionally this storage can get deleted, as losing your Private key(s) (in keypairs) would be catastrophic, from version 3.60,
whenever you create a new key pair, a back-up file (all key pairs) '
keypairs.txt'
is created and downloaded to your downloads folder, also a back-up of your Public keys called '
publickeys.txt'.
Re-installing your key pairs from the back-up file is simple: click on the Kodamail tab (top right in Gmail), then 'Admin, Public/Private Keys Manager',
select 'Your Key Pairs' and then the link: '
Import key pairs (back up of all keys)
Using the 'Choose file' button, select your 'keypairs.txt' file and your key pairs will be re-installed into Local Storage.
You can also reinstall your Public Keys using '
publickeys.txt' by clicking 'Public Keys' and then 'Import Public Keys'.
The private key is always encrypted with your pass phrase,
You will only be asked to decrypt it if you need to decrypt an email or file OR if signing an email.
If you were wanting to run a different version of Chrome, eg Chrome Canary or the Yandex browser, you could use this import method to install your keypairs from your backup: keypairs.txt.
