Login bug in 1.6.1.5 (Leotheme)

[AmirRzd]

There was a weird bug in my Prestashop store (http://smilehyper.ir) which I need to explain step by step.

 

This bug is related to two customers of mine which I call them A and B.

 

When customer A (also my friend) was visiting my website, he saw the name of customer B as logged in. As he was shocked he tried to refresh the page and it made the account to log out. 

 

He told me this and I checked. While this was happening, Customer B was also trying to log in. 

 

Then I checked and the customer A account was deleted (automatically) and 35 carts with different products from different categories with different amounts with no user related to theme were created in a few seconds. Also some button like "login" stopped working. Of course everything is back to normal now.

 

I use Prestashop 1.6.1.5 and Leotheme eMarket theme on a Linux shared-host.

 

What might be the source of this weird bug/problem?

 

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P.S: I posted this on behalf of my friend who owns the shop.