Clear Cookies to Resolve Login Issues
Revert Back to the Default Theme
WordPress themes can also cause conflicts after upgrading to a newer version of WordPress or the theme itself. To find out whether the problem is being caused by your theme, you need to deactivate your theme. The process is similar to deactivating the plugins. Connect to your website using an FTP client. Go to /wp-content/themes/ directory and rename your current theme directory to anything. WordPress will look for your current theme and will fall back to the default theme. Once you have done that, then try logging in again. If you succeed, then this means your theme was causing the issue.