Plugin Stoped working

Hi,

Contact Form 7 forms on my WordPress site no longer submit. After clicking Submit, the loading spinner keeps spinning indefinitely.

The REST API request returns:

POST /wp-json/contact-form-7/v1/contact-forms/72437/feedback

HTTP 400 Bad Request

Response:

{
  "code": "wpcf7_unit_tag_not_found",
  "message": "There is no valid unit tag.",
  "data": {
    "status": 400
  }
}

The submitted payload does contain a valid-looking unit tag:

_wpcf7: 72437
_wpcf7_version: 6.1.7
_wpcf7_locale: de_DE
_wpcf7_unit_tag: wpcf7-f72437-p70140-o1
_wpcf7_container_post: 70140

I also checked the DOM. The wrapper ID and hidden _wpcf7_unit_tag are identical:

Wrapper ID: wpcf7-f72437-p70140-o1
_wpcf7_unit_tag: wpcf7-f72437-p70140-o1

Troubleshooting already performed:

• Disabled WP-Optimize
• Put Wordfence into Learning Mode
• Disabled Conditional Fields for Contact Form 7
• Tested in a private/incognito browser session
• Created a completely new CF7 test form and a new test page

The new test form produces exactly the same wpcf7_unit_tag_not_found error, so the issue does not appear to be specific to the original form.

Contact Form 7 version: 6.1.7

Does anyone know what could cause CF7 to reject _wpcf7_unit_tag even though the unit tag is present in the POST request and matches the form wrapper ID in the DOM?

Thanks!