if username or password is incorrect wp-login returns a blank page

The blank page means there’s a server error, check the error logs, or add the following line to your wp-config.php file to get them to show on screen.

define( 'WP_DEBUG', true );

That will show you the errors so you can start debugging.

OR

It happens because the theme or plugins that you installed are not supported.

How to check and fix:

• Rename the plugin folder name and theme folder name. If opening
  /wp-admin and entering a valid username and password the dashboard is
  shown, then it worked.

• Revert the folder name of plugin and theme, then activate the themes
  and plugins one by one, cross-checking with /wp-admin. You will find
  out the unsupported plugin or theme