Hello @zluck,
I have the same problem after updating all plugins to the latest version.
WP 5.6.2
Contact Form 5.4
MultiLine files for Contact Form 7 Version 1.9
After sending the form I have a Fatal error:
PHP Fatal error: Uncaught Error: Call to private method WPCF7_Submission::add_uploaded_file() from context '' in .../wp-content/plugins/multiline-files-for-contact-form-7/multiline-files-upload-for-contact-form-7.php:467
