If your Tridion webform is running under the SDL CMS website and CMS url having https means you need to have certificate to access the site.
As per my knowledge tridion webform use one "webformWS" webservice and it must be installed some where in IIS.
Please check the webservice named with Webform in your IIS and check the property.
as per your environment it must be running under your SDLCMS website. and they have applied certificate in webservice as well.
Please go and de-activate the certificate from it.
Below are the step(s):
1.) Click Start, point to Administrative Tools, and then click Internet Information Services (IIS) Manager.
2.) In IIS Manager, expand the local computer, and then expand Web Sites.
3.) Right-click virtual folder/ website and then click Properties.
4.) Click the Directory Security tab, and then Server Certificate.
5.) Click on the Edit button under secure communication section.
Click Start, point to Administrative Tools, and then click Internet Information Services (IIS) Manager.
In IIS Manager, expand the local computer, and then expand Web Sites.
Right-click virtual folder/ website and then click Properties.
Click the Directory Security tab, and then Server Certificate.
Click on the Edit button under secure communication section.
a. UnUn-check the “Require secure channel (ssl) option.
b. SelectSelect the “Ignore certificates” option.
6.) Click “OK” and close the box.
- Click “OK” and close the box.
