I have modified the ImportWP PRO plugin on your server, i was unable to use the plugin editor to do this due to an error happening each time i tried to save: "Scrape nonce check failed". I was able to overcome this by reuploading the plugin each time i wanted to make a change.
By adding importer debug logs into the importer before and after each plugin call i was able to narrow down the issue with your server and the white screen problem (Server Error 500) to the core wordpress function "wp_handle_upload" that throws this error.
i can see that the file uploaded from the website to your servers temp direcory based on the $_FILES array showing no error and the file has a size.
I then tried to download a file from a remote source and it got to the core wordpress function "wp_unique_filename" and then errored.
I have setup a fresh local copy of wordpress using openlitespeed and matched your php and database versions, along with installing and activating the same list of plugins you have and was not able to replicate your issue.
Without seeing your servers php error log or WordPress debug log i cant progress any further.