We have managed to pass this generic error or at least understand where it comes from.
When you encounter the "technical reason" error, first of all check your access log.
As I have mentioned before, the app is created in PHP - the place to look for it should be:
tail -50 /var/log/apache2/access.log
"POST {your payment session endpoint set in the payment app extension} HTTP/1.1" 200 3463 "-" "Faraday v1.9.3"
I've build the app on top of Laravel, which has a redirect directive in .htaccess
# Redirect Trailing Slashes If Not A Folder...
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} (.+)/$
RewriteRule ^ %1 [L,R=301]
Because the payment session URL was submitted with a trailing slash and the path was not a folder, apache would respond with 301 instead of 200, which would determine Shopify to throw the error.
This was our particular issue, but basically everything that goes wrong when Shopify makes the request to the payment session URL, it will result in the same message. What will make your life easier is to check the server access logs, server error logs or even better write your custom logs in the app. This request is not visible in browser.
Happy coding with Shopify!
