Invalid Token Format

Hi all
I have a tricky little problem: for the last few weeks when I log in to the aditazz.com admin area, I get an error message in the browser (the message is posted at the bottom of this topic in italics).
When I hit refresh twice, I proceed to the site admin area.
When I de-activate Gmail SMTP, I don’t get the error message and can proceed straight to the admin area.
But, on the page posted above about halfway down with regards to the Download Paper button, we’re using Gmail SMTP in conjunction with Zapier and Copper to allow the visitor to download the paper (sent to them in an email) while we capture the details of the visitor. This process has been working fine for about 18 months.
Does anybody have an idea what might be happening here?

Warning: Illegal string offset ‘refresh_token’ in /var/www/html/wp-content/plugins/gmail-smtp/main.php on line 685

Fatal error: Uncaught InvalidArgumentException: Invalid token format in /var/www/html/wp-content/plugins/gmail-smtp/google-api-php-client/src/Google/Client.php:434 Stack trace: #0 /var/www/html/wp-content/plugins/gmail-smtp/class.phpmaileroauthgoogle.php(145): Google_Client->setAccessToken(‘{“access_token”…’) #1 /var/www/html/wp-content/plugins/gmail-smtp/PHPMailer/class.smtp.php(475): GmailXOAuth2->getOauth64() #2 /var/www/html/wp-content/plugins/gmail-smtp/class.phpmaileroauth.php(152): SMTP->authenticate(”, ”, ‘XOAUTH2’, ”, ”, Object(GmailXOAuth2)) #3 /var/www/html/wp-content/plugins/gmail-smtp/PHPMailer/class.phpmailer.php(1542): PHPMailerOAuth->smtpConnect(Array) #4 /var/www/html/wp-content/plugins/gmail-smtp/PHPMailer/class.phpmailer.php(1337): PHPMailer->smtpSend(‘Date: Fri, 18 J…’, ‘This email was …’) #5 /var/www/html/wp-content/plugins/gmail-smtp/PHPMailer/class.phpmailer.php(1215): PHPMailer->postSend() #6 /var/www/html/wp-content/plugins/gmail-smtp/main.php(943): PHPMailer->send() #7 /var/www/html/wp- in /var/www/html/wp-content/plugins/gmail-smtp/google-api-php-client/src/Google/Client.php on line 434

The page I need help with: [log in to see the link]