File uploading problem

Error code 7 is a PHP error see here: https://www.php.net/manual/en/features.file-upload.errors.php

Never had this reported. Some posts online indicate it happens more with larger files so it could be the server giving up on writing it at some point. Maybe reaching a process limit.

You can upload .csv file using FTP and then use it the plugin. I recommend testing a small .csv file first and see how you get on. Look in the wp-content folder to find the directory where you .csv files will be stored.