I don't know what link you're providing to the users....I assume just the direct link as found in the "embed" section which is just a direct link, to the empty, new form.
The Save and Resume function is not used to start a new form, it's for an existing incomplete submission, and the link is only valid until the form is fully and completely submitted. The link then expires since it's of no further use for that particular submission record.
So I don't know what's going on.
Do you need the Save and Resume feature? To me when talking of interviews that suggests it will be completed, and submitted during that interview; it doesn't seem logical that it will need to be saved and submitted at a later time.
Perhaps it is better that you contact Machform directly and submit a support ticket (the forum is not for official support). http://discuss.machform.com/d/1-this-is-community-forum-use-our-ticket-system-for-support