PayPal merchant and session destruction

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Hi there! I wonder if somebody came across with the next situation:
After the payment flow, I click the link "Return to Merchant" 

and I go back to my website but unfortunately my session is destroyed. 

Did somebody face an identical situation?

Update: I've made an empty form 

 , I mean that it works on its own without any engine's script. And the problem is in existence. 2-3 times out of 10 it works incorrectly. I can't catch a cause. When the problem occurs, the browser doesn't send cookies 

  and sends if works