Hi @iowebgr
It sounds like you are experiencing an issue with the checkout success page not displaying correctly after upgrading to Magento 2.3.1 CE and trying different one step checkout solutions.
Based on your description, it seems like the issue might be related to the checkout session data being empty by the time it reaches the success controller, causing the Checkout SuccessValidator to fail and redirecting to the cart page with an empty cart.
To track down the issue, you could try setting breakpoints in the code to step through the checkout workflow and see where the checkout session data is being reset or reinitialized. You could also try reviewing the Magento 2.3.1 release notes and documentation to see if there are any known issues or changes to the checkout process that could be causing the problem.
Additionally, you may want to check with the developers of the one step checkout solutions you have tried to see if they have any suggestions or updates that could address the issue. It could be a compatibility issue with Magento 2.3.1 or a bug that needs to be fixed.
Overall, it will require some investigation and troubleshooting to determine the root cause of the problem and find a solution.
Give me a kudos if this help
