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Are you using the Yola Form Widget for your form? If so, the subject of the email comes from the form heading. If you click on the Form in Sitebuilder and it opens the Form Edit Window, the top line of text under the "Edit" tab is the subject of the email message. If you click on it, it will allow you to change it to something else.
Perhaps even if it says what it should, try changing it to something else and then change it back to what you want it to say. This should reset it.