Yes, as Richard recommends, removing the offending account and then setup again often fixes the problem.
When you setup the account through the Automatic Setup, eM Client used GMail’s OAuth process, if you did not enter the password correctly, you would not have reached the success screen.
But later if you change your GMail password, it is not required to change it in eM Client. In fact you can’t change it in eM Client. The Google server has a token to allow eM Client to access your GMail account, it is not actually a password. You can change your GMail password daily through the web interface and eM Client will just carry on connecting with the token.
If for some reason you want to go through the OAuth process again, just to make sure it is working, go to https://myaccount.google.com/security#connectedapps and delete eM Client from the connected apps.
The next time eM Client tries to sync with GMail, you will go through the OAuth process again.
