Contact Form 7 version 5.4 was released on February 24, 2021. It contains a lot of breaking changes which cause many issues with PDF Forms Filler for Contact Form 7. We are currently working on making PDF Forms Filler for Contact Form 7 compatible with the new version of Contact Form 7. Meanwhile, the solution at this time is to downgrade to a previous version of Contact Form 7 (version 5.3.2). Please take care to not delete your existing forms when you perform the downgrade (don’t delete the Contact Form 7 plugin). You can use the WP Rollback plugin to do the downgrade. Be sure to disable auto-updates for the Contact Form 7 plugin to avoid it being automatically updated again.

The release announcement post by the developer of Contact Form 7 goes into more details about what changed. The changes that affect PDF Forms Filler for Contact Form 7 are as follows:
1. The hiding of add_uploaded_file() function.
2. Changed output from WPCF7_Submission::uploaded_files().
3. Download link feature is affected (plain text version of the response message is now shown instead of the HTML version)

We are going to be releasing a new version with compatibility fixes once they become available.