After debugging this problem, I found this error in the PHP error log located at public_html/mywebsite/wp-admin/error_log:
PHP Fatal error: Call to undefined function wp_raise_memory_limit() in /home/example/public_html/mywebsite/wp-admin/admin.php on line 141
This function wp_raise_memory_limit() should be defined in wp-includes/functions.php but it is not defined in my case due to an unsuccessful update by WordPress.
So Solution: Do a manual update for wordpress as I did the following:
- I downloaded the latest version of wordpress from wordpress.org
- I replaced all files and folders of my site except
wp-config.phpfile andwp-contentfolder - I opened my site url, it requested a database update
I pressed on update database and the problem resolved 🙂