Blank Page / HTTP Error 500 / Internal Server Error
An HTTP Error 500 - also known as Internal Server Error - tells you that something went wrong on your server, something which couldn't be handled in a way that would show a meaningful error message to the browser.
There are various scenarios which could lead to this - from exceeding PHP memory or execution time limits to issues caused by other plugins or software like ModSecurity installed on your server. The first thing to check if you receive a HTTP Error 500 / Internal Server Error problem should be to view your WooCommerce -> System Status page and if your WP Memory Limit is less than 256M then increase the memory limit (see link for instructions on how to increase your PHP memory below).
It greatly depends on your server configuration what happens when PHP runs into any limits or errors - there are servers which show a meaningful error to the browser, some servers simply show a blank page (which is technically an error 500 as well) while some show an actual 500 error message.
Whatever your server is configured to do, whenever PHP runs into a fatal error or out of resources or if anything else happens that results in an error 500, your web server (apache / nginx) will write an error message to its error_log. The location of the actual error_log might vary from server to server, but its impossible for Apache to throw an error 500 without something in its error_log (unless you intentionally disabled all logging).
So please check your error_log for any fatal PHP errors - which look like these:
Fatal error: Call to undefined function missing_function() in /www/website/wp-content/plugins/some-plugin/plugin.php on line 123.
Fatal error: Allowed memory size of 654321 bytes exhausted (tried to allocate 12345 bytes) in /www/website/wp-content/plugins/some-plugin/plugin.php on line 123.
If you see an error about memory size, please follow the instructions about Increasing the PHP memory limit on your server.
If you see an error pointing to a 3rd party plugin, please try to disable the plugin and contact its developer.
A plugin as complex as resource intensive as WP-Lister and the importer add-on can't possibly work on every server and web hosting - nor with every theme and other plugin there is. We have often witnessed poorly coded plugins and themes breaking the import process or the user interface, so the quickest way to identify the cause would be to disable all other plugins except WP-Lister and WooCommerce and see if everything works as expected then.
If that doesn't help and nothing is to be found in your error_log, you might have to change to a better hosting provider - or contact us within 30 days after your purchase to get a full refund.