The latest version of Event Espresso 4 has a new batched registration CSV report system. This means that batches of records are retrieved in the background and then combined into one file. This is really helpful for events that have hundreds or thousands of registrants/attendees.
If your site is having trouble with the new system, then you can temporarily switch back to the old system.
An SFTP or FTP client will be needed. FileZilla and Cyberduck are free options that are available for multiple platforms. Using a Mac? Try Transmit.
Login to your WordPress site using your preferred SFTP / FTP client and browse to the WordPress root.
Then open your wp-config.php file for editing. Copy the following constants:
define( 'EE_USE_OLD_CSV_REPORT_CLASS', true );
Then paste then towards the middle of your wp-config.php and save changes. From here, return to your WordPress dashboard (WP-admin) and try the generating the registration CSV report again.
References:
https://eventespresso.com/topic/ee4-export-to-csv-after-update-to-4-8-26p-no-file-generated/
https://eventespresso.com/topic/i-am-unable-to-export-registrations-for-all-my-events/
https://eventespresso.com/topic/reports-not-working-2/