Hi @shadabdurrani
This issue is related to cookies. You can check and increase cookies lifetime from here in Magento:
Sytem -> Configuration -> Web -> Session and Cookie Management
The Reason that I found out after doing some research on internet and different forum is that in Magento, by default cookie lifetime is set to 3600 (1 hour). But if the end users computer time runs ahead of server’s time, cookies will not get set for Magento frontend as well as backend. For example, end user’s computer time is 1 hour forward than server’s time, that means the cookie (holding user’s session id) will expire as soon as user logs in or tries to add an item.
OR
Here we have some links might be helpful for you. Link1 Link2
I hope it will help you.
Thanks
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