Using HTML snippets for Forms no longer an option?
Looking at the release notes for CiviCRM 4.7.11 there was a security issue that was fixed to do with these external snippets: https://civicrm.org/advisory/civi-sa-2016-17-manage-csrf-overrides-for-external-profile-forms

The release notes state, though, that existing installations should not have been affected by the update.

I would recommend that you look in the settings (Administer => System Settings => Misc) for the QF key security on profile forms.

If this setting has been enabled then it sounds like a bug - as the release notes suggest it should not have been enabled when upgrading an existing installation. If so then report it on the issue log and comment back here with a link =]

It is certainly worth reviewing that security issue though, which discusses the implications of using HTML snippets (and offers some preferable alternatives).