The problem has nothing to do with Marketo nor Marketo forms -- your page as a whole doesn't load on Windows 7/IE11. (Perhaps the people reporting problems said "your form doesn't render" but that's burying the lede!) The SSL error is because your web team is not delivering an acceptable SSL certificate chain for the Windows 7 certificate store. They assumed the CA "Entrust Certification Authority L1K" would be automatically trusted even in Windows 7. In fact, the next-up issuer "Entrust.net Certification Authority (2048)" is trusted in Windows 8 and later (meaning: it's either installed in the original OS or gets updated as part of regular Windows Updates) but not Windows 7. So they have to package a new cert chain that ends with a CA that's part of the Windows 7 cert store.
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