Error on log insert

Getting the following error when email logging is enabled:
PHP Recoverable fatal error: Object of class WP_Error could not be converted to string in /home/yogameditation/public_html/wp-content/plugins/post-smtp/Postman/Postman-Email-Log/PostmanEmailLogService.php on line 154
Logging seems to work anyway but it causes the server to respond 500 which breaks the ajax response when submitting a form (I am using contact form 7)

Grateful for any insight into this