Hm, strange. I’ve imported your backup and it works fine for me.
So, you’re selecting your backup, the button changes to that spinning circle, the page reloads and what happens next? Does the plugin produce an error message?
Maybe your server/PHP is producing some errors and since WP is not showing them by default you could set some constants inside your wp-config.php:
define("WP_DEBUG", true);
define("WP_WP_DEBUG_DISPLAY", false);
define("WP_DEBUG_LOG", true);
Have a look at Debugging in WordPress for some explanation what those lines are doing.
The article is not up to date about WP_DEBUG_LOG: Miscellaneous Developer Focused Changes in 5.1.
When you’re done don’t forget to set WP_DEBUG back to false.