At what moment did you meet the error? Did you can select file to import and click “Import” button?
Try to turn on WordPress debugging to see the PHP error on screen, if you have any. You may do it inserting this string into wp-config.php
Thanks for your help. I will fix this error with the next update, I hope it will be published this weekend.
Plugin tries to notify you about problem with file upload. Please check that wp-content/user-role-editor-pro folder is writable for your web server. It should have 775 permissions usually.
Plugin uploads roles file to this folder, reads data from it and then deletes that file.