Fatal error after setting up Gmail client

Replies: 0

I am having the same error as #post-13382766:

Fatal error: Uncaught Error: Call to undefined function GuzzleHttp\choose_handler() in /var/www/vhosts/mydomain/httpdocs/wp-content/plugins/wp-mail-smtp/vendor/guzzlehttp/guzzle/src/HandlerStack.php:42

Doing a search inside my wp-content directory for the file in question (HandlerStack.php) gives me these results:

./wp-content/plugins/wpdm-dropbox/libs/vendor/guzzlehttp/guzzle/src/HandlerStack.php
./wp-content/plugins/wordpress-seo/vendor_prefixed/guzzlehttp/guzzle/src/HandlerStack.php
./wp-content/plugins/w3-total-cache/lib/Aws/GuzzleHttp/HandlerStack.php
./wp-content/plugins/w3-total-cache/lib/Azure/GuzzleHttp/HandlerStack.php
./wp-content/plugins/amazonsimpleadmin/vendor/asaguzzlehttp/guzzle/src/HandlerStack.php
./wp-content/plugins/wp-mail-smtp/vendor/guzzlehttp/guzzle/src/HandlerStack.php

All these plugins are active except W3-Total-Cache.