I seem to be getting this message after i updated to Magento 2.1.7 - it only displays on the contact page when someone submits the form.
Invalid response line returned from server: HTTP/2 200
I see this in the magento log file:
main.CRITICAL: Notice: Array to string conversion in /home/xxx/public_html/vendor/magento/module-catalog/Block/Product/View/Attributes.php on line 83 [] []