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In order to make your ligtbox form secure it is not enough by changing the http for the https protocol of the url, could you try by generating the lightbox code again checking the secure form option.
From your form builder:
1. Click on "Setup & Embed" tab.
2. Click on "Embed Form"
3. Check the "Secure form" option
4. Generate the Lightbox code
Hope it helps
please let us know if you need further assistance