Gmail + UpdraftPlus + Post SMTP + attachment fail

Email notification from UpdraftPlus plugin –> Post SMTP (Gmail) is failing. Suspect culprit is small txt file (activity log) attached to email. Note: email send succeeds with Post SMTP disabled.

WP 5.0.3, Post SMTP 1.9.6

Debug log:
[14-Feb-2019 20:40:17 UTC] PHP Fatal error: Uncaught TypeError: Argument 2 passed to Google_Http_MediaFileUpload::__construct() must be an instance of Psr\Http\Message\RequestInterface, instance of Google_Http_Request given, called in /home/xzyzxx/public_html/mydomain.org/blog/wp-content/plugins/post-smtp/Postman/Postman-Mail/PostmanGmailApiModuleZendMailTransport.php on line 211 and defined in /home/xzyzxx/public_html/mydomain.org/blog/wp-content/plugins/post-smtp/Postman/Postman-Mail/google-api-client/vendor/google/apiclient/src/Google/Http/MediaFileUpload.php:76
Stack trace:
#0 /home/xzyzxx/public_html/mydomain.org/blog/wp-content/plugins/post-smtp/Postman/Postman-Mail/PostmanGmailApiModuleZendMailTransport.php(211): Google_Http_MediaFileUpload->__construct(Object(Google_Client), Object(Google_Http_Request), ‘message/rfc822’, ‘X-Mailer: Postm…’, true, 1048576)
#1 /home/xzyzxx/public_html/mydomain.org/blog/wp-content/plugins/post-smtp/Postman/Postman-Mail/Zend-1.12.10/Mail/Transport/Abstract.php(348): Postman in /home/xzyzxx/public_html/mydomain.org/blog/wp-content/plugins/post-smtp/Postman/Postman-Mail/google-api-client/vendor/google/apiclient/src/Google/Http/MediaFileUpload.php on line 76

Ideas anyone? Thanks for your kind help! 🙂

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