Error When Create/Update Post

Hi,
I have use your plugin for 6 Month. I use WordPress 5.4.2. I have created a new post everyday and dont have any of error.
But, today when I create a new/update post, I got this error message :

Fatal error: Uncaught Google_Service_Exception: {“error”:”invalid_grant”,”error_description”:”Invalid grant: account not found”} in /usr/local/lsws/Example/fluttercore.com/wp-content/plugins/fast-indexing-api/vendor/google/apiclient/src/Google/Http/REST.php:118 Stack trace: #0 /usr/local/lsws/Example/fluttercore.com/wp-content/plugins/fast-indexing-api/vendor/google/apiclient/src/Google/Http/REST.php(94): Google_Http_REST::decodeHttpResponse(Object(GuzzleHttp\Psr7\Response), Object(GuzzleHttp\Psr7\Request), NULL) #1 /usr/local/lsws/Example/fluttercore.com/wp-content/plugins/fast-indexing-api/vendor/google/apiclient/src/Google/Task/Runner.php(176): Google_Http_REST::doExecute(Object(GuzzleHttp\Client), Object(GuzzleHttp\Psr7\Request), NULL) #2 /usr/local/lsws/Example/fluttercore.com/wp-content/plugins/fast-indexing-api/vendor/google/apiclient/src/Google/Http/REST.php(58): Google_Task_Runner->run() #3 /usr/local/lsws/Example/fluttercore.com/wp-content/plugins/fast-indexing-api/vendor/google/apiclient/src/Google/Client.php(7 in /usr/local/lsws/Example/fluttercore.com/wp-content/plugins/fast-indexing-api/vendor/google/apiclient/src/Google/Http/REST.php on line 118

When I disable this plugin (Instant Indexing), no error showed. So, I think the error related with this plugin.
Thank you very much.
Best regards,

Toto