Here is the error I get from the logs
Fatal error: Uncaught Error: Call to undefined function GuzzleHttp\Psr7\uri_for()
…/htdocs/wp-content/plugins/fluent-smtp/includes/libs/google-api-client/vendor/google/auth/src/OAuth2.php on line 1274
Call stack:
Google\Auth\OAuth2::coerceUri()
wp-content/plugins/fluent-smtp/includes/libs/google-api-client/vendor/google/auth/src/OAuth2.php:679
Google\Auth\OAuth2::setAuthorizationUri()
wp-content/plugins/fluent-smtp/includes/libs/google-api-client/vendor/google/auth/src/OAuth2.php:334
Google\Auth\OAuth2::__construct()
wp-content/plugins/fluent-smtp/includes/libs/google-api-client/vendor/google/apiclient/src/Google/Client.php:942
Google_Client::createOAuth2Service()
wp-content/plugins/fluent-smtp/includes/libs/google-api-client/vendor/google/apiclient/src/Google/Client.php:922
Google_Client::getOAuth2Service()
wp-content/plugins/fluent-smtp/includes/libs/google-api-client/vendor/google/apiclient/src/Google/Client.php:274
Google_Client::fetchAccessTokenWithRefreshToken()
wp-content/plugins/fluent-smtp/includes/libs/google-api-client/vendor/google/apiclient/src/Google/Client.php:255
Google_Client::refreshToken()
wp-content/plugins/fluent-smtp/app/Services/Mailer/Providers/Gmail/Handler.php:265
FluentMail\A\S\M\P\G\Handler::getApiClient()
wp-content/plugins/fluent-smtp/app/Services/Mailer/Providers/Gmail/Handler.php:59
FluentMail\A\S\M\P\G\Handler::sendViaApi()
wp-content/plugins/fluent-smtp/app/Services/Mailer/Providers/Gmail/Handler.php:24
FluentMail\A\S\M\P\G\Handler::postSend()
wp-content/plugins/fluent-smtp/app/Services/Mailer/Providers/Gmail/Handler.php:15
FluentMail\A\S\M\P\G\Handler::send()
wp-content/plugins/fluent-smtp/app/Services/Mailer/FluentPHPMailer.php:28
FluentMail\A\S\M\FluentPHPMailer::send()
wp-content/plugins/fluent-smtp/app/Functions/helpers.php:561
fluentMailSend()
wp-content/plugins/fluent-smtp/fluent-smtp.php:40
wp_mail()
wp-includes/user.php:2609
wp_update_user()
wp-admin/includes/user.php:227
edit_user()
wp-admin/user-edit.php:167