Magento 2 Checkout Problem

Checkout Problem.

When we try to place an order and visit the one step checkout page the system redirects us to the account page or log out and we have to put the details from the beginning.

We need someone to start working after he will accept the order and deliver it as soon as possible.
Before we accept any proposals we would like to know for sure the source of the problem and how it can be fixed. Also the solution it is important will not cause any other conflicts with extensions or website performance.