SSL Post not submitting form with SSL forced

I’m using Gravity Forms on many posts on this site. Here is a test post with a form:

I’m using WordPress HTTPS to force SSL on these posts like this. When I try to submit an incomplete form, 2 things go wrong:
1st – FireFox throws up a dialog stating:

Although this page is encrypted, the information you have entered is to be sent over an unencrypted connection and could easily be read by a third party.

Are you sure you want to continue sending this information?

2nd – the form is not submitted and the page refreshes.

I’ve tested this page and the form works correctly when the page is not forced to SSL.
If I enter the correct information in the form and submit it while it is SSL forced, the dialog comes up but the form is processed.

Can you help?
Thanks! Chris