This problem occurred on GoDaddy hosting, but could also occur on other web hosts.
Problem Description:
Unable to access some WordPress pages. Plugins, Updates, etc.
When trying to access these pages you are redirected and a 404 error occurs.
If you are checking with Google Chrome Developer Tools (Console Tab) you will see 404 errors for the /wp-admin/admin-ajax.php file.
Cause of the problem:
Corrupt Session file in the server’s /tmp folder.
Solution/Fix:
To fix the problem login to your hosting account > go to cPanel > open the File Manager tool > go to the tmp folder (located in the left hand folder column) > delete all Session files.