If this meassage appears directly when you open the form, then there is a damage in the template, so Designer could not read it.
This sometime happens if the forms were mailed and the compression of the mail application destroyed the form.
Can you still open the form when you confirm the message?
And, does the form open in Acrobat or Reader?
If you like you can send me the form and I'll take a look on it.
Just send me a private message or post a link where I can download it.