Debug/fix Prestastashop 1.7 multistore's payments not working

I'm working on a multistore Prestashop 1.7 and had installed a module that would make the normal checkout easier. But it didn't work (I thought) so I uninstalled it. I have tried to get payment methods to work after that but they don't work, I don't know if they did before the module either.

- I have installed a payment module (Klarna checkout) that does an express checkout on a seperate url, this works fine (I thought) - except the order is not found in admin afterwards.
- I have installed wire payment module (prestashop's) and check module (prestashop's) - one of them is needed and the other I installed just to see if I could get any of them to work. They don't work, I get error on validation page. However for some reason some of the orders still show up in admin afterwards.
- I also have installed a paypal, which doesn't have the button show up at all (perhaps connected to validation or something i don't know).


What I need:
So, the paypal module - I don't need debug/fix for this as I can take that later on with the module creator if the issue with that one still persist.
The Klarna checkout needs to work, and the "normal" prestashop checkout page needs to work (with bank wire module). So I need help debugging why it doesn't work - and fix it so that it works.


Some error logs:
PaymentModule::validateOrder - Secure key does not match
When uninstalling the easy checkout: Exception thrown by module easypay on uninstall. Warning: A non-numeric value encountered

Requirements:
- A lot of experience with Prestashop 1.7
- Be able to do this within the next couple of days
- Be able to solve the issues without Skype phone-call (I'm able to do text-chat via Skype if neccessary)