After a lot of search for how to solve this problem, her what I found to help in such a situation:
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Enabling the Debug mode & display errors and warnings
// Enable WP_DEBUG mode define('WP_DEBUG', true); // Enable Debug logging to the /wp-content/debug.log file define('WP_DEBUG_LOG', true); // Display errors and warnings define('WP_DEBUG_DISPLAY', true);
at this point i hope some error message appears so can help identify the problem.
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Some time no indication of what is the cause of the problem (like my case), so I changed the plugins folder name to anything (e.g. plugins.test) then try to log-in to the admin section again.
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Now the admin section is working, so the problem was in one of the plugins.
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To identify the plugin causing the problem, rename the plugins folder again to its original name but change plugins that you have inside the folder.
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One at a time rename each plugin folder and test to login to the admin section.
For me, it was because of the gravity form plugin which causes the problem. I updated the plugin then admin section works. Hope this will help anyone.