WebForm: Access Denied after submitting WebForm – after update to Drupal 7.77

We’re running on Drupal 7.73 and WebForm 7.x-3.30 – for a while now since the 7.75 to 7.77 Drupal release, we’ve been unable to update Drupal successfully.
As every time a user submits a form on the site, we’re getting an “Access Denied” message.
If you’re logged in as Admin it works. I believe it could have something to do with the “success message” that’s being displayed and that in theory, a redirect to a success page could fix it.
However, as we have many forms across several sites, it is not practical to go this route.

For two years we have had no problem updating Drupal to latest and no issues with WebForm so this is a new issue.

Can you please assist in providing a solution for this as we are unable to update to implement latest security fixes?