Hi,
This means that there is a fatal error on the page. By activating the "error reporting" setting of the Joomla configuration you should see the error message on that blank page. And we might be able to help you fix it. But then, you might just get another fatal error. And you should have many other problems here and there, including on the frontend as the 2.1.3 isn't supposed to work neither with Joomla 3.9 nor with PHP 7.4.
The update to the 4.2.3 should have less important issues than the 2.1.3 so I would still recommend updating even if you have small issues after the update.