We recently ran into a very difficult bug to diagnose. A customer will browse the website, add items to their cart, go to the checkout, and select Payline credit card. When they press the final "Place Order" button on the onepage checkout screen, they will get directed to "GET /payline/index/cpt/", but then randomly get redirected to the homepage. Their entire session will be lost and they will need to completely redo the checkout process again. We thought it was an issue with the Payline checkout screen so we setup a ton of debugging in the payline script itself, but found out that the module is never hit when the redirect is triggered. The main observer is not loaded from what we can tell. The user just gets redirected to the homepage and their cart/session is lost).
The redirect SHOULD go to the Payline secure online checkout screen (similar to PayPal).
All the items we have checked:
Session Validation Disabled
Form key disabled
Disabled optimization on the onepage checkout
No discernible pattern of browsers: Chrome/Firefox on Mobile/Desktop/Tablets, on Laptops/tablets/mobile phones.
No errors in any of the Magento logs (exception/system) nor in any of the webserver logs.
The question would be what steps should we try to trace the calls to the module. Any other suggestions as to what else the issue might be?