Exception when saving post

Here is the stack trace :

PHP Fatal error: Uncaught DomainException: OpenSSL unable to sign data in /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/firebase/php-jwt/src/JWT.php:239

Stack trace:

#0 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/firebase/php-jwt/src/JWT.php(202): Firebase\JWT\JWT::sign('eyJ0eXAiOiJKV1Q...', '-----BEGIN PRIV...', 'RS256')

#1 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/google/auth/src/OAuth2.php(470): Firebase\JWT\JWT::encode(Array, '-----BEGIN PRIV...', 'RS256', NULL)

#2 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/google/auth/src/OAuth2.php(504): Google\Auth\OAuth2->toJwt()

#3 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/google/auth/src/OAuth2.php(544): Google\Auth\OAuth2->generateCredentialsRequest()

#4 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/google/auth/src/Credentials/ServiceAccountCredentials.php(205): Google\Auth\OAuth2->fetchAuthToken(Object(Google\Auth\HttpHandler\Guzzle7HttpHandler))

#5 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/google/auth/src/FetchAuthTokenCache.php(74): Google\Auth\Credentials\ServiceAccountCredentials->fetchAuthToken(Object(Google\Auth\HttpHandler\Guzzle7HttpHandler))

#6 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/google/auth/src/Middleware/AuthTokenMiddleware.php(122): Google\Auth\FetchAuthTokenCache->fetchAuthToken(Object(Google\Auth\HttpHandler\Guzzle7HttpHandler))

#7 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/google/auth/src/Middleware/AuthTokenMiddleware.php(102): Google\Auth\Middleware\AuthTokenMiddleware->fetchToken()

#8 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/guzzlehttp/guzzle/src/PrepareBodyMiddleware.php(64): Google\Auth\Middleware\AuthTokenMiddleware->Google\Auth\Middleware\{closure}(Object(GuzzleHttp\Psr7\Request), Array)

#9 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/guzzlehttp/guzzle/src/Middleware.php(31): GuzzleHttp\PrepareBodyMiddleware->__invoke(Object(GuzzleHttp\Psr7\Request), Array)

#10 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/guzzlehttp/guzzle/src/RedirectMiddleware.php(71): GuzzleHttp\Middleware::GuzzleHttp\{closure}(Object(GuzzleHttp\Psr7\Request), Array)

#11 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/guzzlehttp/guzzle/src/Middleware.php(61): GuzzleHttp\RedirectMiddleware->__invoke(Object(GuzzleHttp\Psr7\Request), Array)

#12 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/guzzlehttp/guzzle/src/HandlerStack.php(75): GuzzleHttp\Middleware::GuzzleHttp\{closure}(Object(GuzzleHttp\Psr7\Request), Array)

#13 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/guzzlehttp/guzzle/src/Client.php(331): GuzzleHttp\HandlerStack->__invoke(Object(GuzzleHttp\Psr7\Request), Array)

#14 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/guzzlehttp/guzzle/src/Client.php(107): GuzzleHttp\Client->transfer(Object(GuzzleHttp\Psr7\Request), Array)

#15 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/guzzlehttp/guzzle/src/Client.php(123): GuzzleHttp\Client->sendAsync(Object(GuzzleHttp\Psr7\Request), Array)

#16 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/google/auth/src/HttpHandler/Guzzle6HttpHandler.php(47): GuzzleHttp\Client->send(Object(GuzzleHttp\Psr7\Request), Array)

#17 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/google/apiclient/src/Http/REST.php(82): Google\Auth\HttpHandler\Guzzle6HttpHandler->__invoke(Object(GuzzleHttp\Psr7\Request))

#18 [internal function]: Google\Http\REST::doExecute(Object(GuzzleHttp\Client), Object(GuzzleHttp\Psr7\Request), NULL)

#19 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/google/apiclient/src/Task/Runner.php(187): call_user_func_array(Array, Array)

#20 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/google/apiclient/src/Http/REST.php(65): Google\Task\Runner->run()

#21 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/google/apiclient/src/Client.php(920): Google\Http\REST::execute(Object(GuzzleHttp\Client), Object(GuzzleHttp\Psr7\Request), NULL, Array, NULL)

#22 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/google/apiclient/src/Http/Batch.php(138): Google\Client->execute(Object(GuzzleHttp\Psr7\Request))

#23 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/includes/class-instant-indexing.php(453): Google\Http\Batch->execute()

#24 /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/includes/class-instant-indexing.php(1096): RM_GIAPI->send_to_api(Array, 'update', false)

#25 /home/xxxxxxxxx/public_html/wp-includes/class-wp-hook.php(326): RM_GIAPI->publish_post(6383, Object(WP_Post))

#26 /home/xxxxxxxxx/public_html/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters(NULL, Array)

#27 /home/xxxxxxxxx/public_html/wp-includes/plugin.php(517): WP_Hook->do_action(Array)

#28 /home/xxxxxxxxx/public_html/wp-includes/post.php(4817): do_action('save_post_post', 6383, Object(WP_Post), true)

#29 /home/xxxxxxxxx/public_html/wp-includes/post.php(4930): wp_insert_post(Array, true, false)

#30 /home/xxxxxxxxx/public_html/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php(880): wp_update_post(Array, true, false)

#31 /home/xxxxxxxxx/public_html/wp-includes/rest-api/class-wp-rest-server.php(1230): WP_REST_Posts_Controller->update_item(Object(WP_REST_Request))

#32 /home/xxxxxxxxx/public_html/wp-includes/rest-api/class-wp-rest-server.php(1063): WP_REST_Server->respond_to_request(Object(WP_REST_Request), '/wp/v2/posts/(?...', Array, NULL)

#33 /home/xxxxxxxxx/public_html/wp-includes/rest-api/class-wp-rest-server.php(439): WP_REST_Server->dispatch(Object(WP_REST_Request))

#34 /home/xxxxxxxxx/public_html/wp-includes/rest-api.php(428): WP_REST_Server->serve_request('/wp/v2/posts/63...')

#35 /home/xxxxxxxxx/public_html/wp-includes/class-wp-hook.php(324): rest_api_loaded(Object(WP))

#36 /home/xxxxxxxxx/public_html/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters('', Array)

#37 /home/xxxxxxxxx/public_html/wp-includes/plugin.php(565): WP_Hook->do_action(Array)

#38 /home/xxxxxxxxx/public_html/wp-includes/class-wp.php(418): do_action_ref_array('parse_request', Array)

#39 /home/xxxxxxxxx/public_html/wp-includes/class-wp.php(813): WP->parse_request('')

#40 /home/xxxxxxxxx/public_html/wp-includes/functions.php(1336): WP->main('')

#41 /home/xxxxxxxxx/public_html/wp-blog-header.php(16): wp()

#42 /home/xxxxxxxxx/public_html/index.php(17): require('/home/xxxxxx...')

#43 {main}

thrown in /home/xxxxxxxxx/public_html/wp-content/plugins/fast-indexing-api/vendor/firebase/php-jwt/src/JWT.php on line 239