We often see this error when Wordfence is installed and it blocks something that it thinks is a potentially dangerous action. The best way to resolve this is to teach the firewall that this action is safe which can be done by taking the following steps:
  1. Sign into your site
  2. Navigate to Wordfence > Firewall
  3. Click the dropdown for Firewall Status and change it from Enabled and Protecting to Learning Mode
  4. Click Save
  5. From here, go back to your form and submit an entry. You should see it go through without any issues.
  6. Repeat steps 2-4 and change the mode back to Enabled and Protecting.